Textualism is a species of originalism.  As such, it is a theory of legal interpretation, that is, a theory about how judges should go about determining the meaning of a legal text, and it could operate at any one of these three levels of interpretation. That is, textualism could be a  view that what legal texts really mean; it could be a view about how judges ought to interpret legal texts; and it could be a practical technique—a method--for  judges to use in interpreting a text. 

The Plain Meaning of the Text
What do we mean when we say “the plain meaning of the text.” A really good answer to that question would requires a theory of meaning in general, but such an answer takes us deep into the philosophy of language and is beyond the discussion here. We can work with an idea of plain meaning is pretty simple. The plain meaning of a legal text is the meaning that would be understood by regular folks who knew that they were reading a statute (or court decision, etc.).

But this preliminary formulation is too simple. Some laws are meant for all citizens (e.g., criminal statutes) and some are meant only for specialists (e.g., some sections of the tax code). A text that means one thing in a legal context, might mean something else if it were in a technical manual or a novel. So the plain meaning of a legal text is something like the meaning that would be understood by competent speakers of the natural language in which the text was written who are within the intended readership of the text and who understand that the text is a legal text of a certain type.

An Brief Inquiry into Meaning
This idea of plain meaning assumes a distinction that can be formulated in terms of the difference between speaker’s meaning and sentence meaning. The speaker’s meaning of a given utterance (or author’s meaning of a given text) is the meaning that the speaker intended the audience to glean for the utterance (or text).

In other words, when someone speaks or writes for a particular audience on a particular occasion, the speaker or author can take into account what she knows about the audience, what the audience knows about her, but only insofar as the speaker knows that the audience knows what the speaker knows about the audience.

Speaker’s meaning can be distinguished from sentence meaning. Sentence meaning is the meaning that an utterance has when the audience is unaware of the speaker’s intentions. When we identify sentence meaning, it is as if we were imaging a sot of generic speaker, who uttered the sentence in a generic context. Or putting this a bit differently, sentence meaning is the meaning we would assign sentences when we know that the speaker can’t assume that we are aware of special conventions about meaning.

Legal Texts, Sentence Meaning & Speaker’s Meaning
Legal texts are sometimes intended for a timeless, generic readership. The authors of legislation, for example, know that many different actors (judges, lawyers, administrative officials, and ordinary citizens) will read the statute for an extended period of time in a variety of different conditions. Many of these readers will not be able to afford access to analysis of the legislative history of the statute; they will simply read the statute itself. Of course, they will know that the text they are reading is a statute, and they will therefore have a fair amount of knowledge about the likely intended meaning of various terms and phrases. Moreover, the legislature knows that the readers of statutes will have this knowledge. So it might make sense to assume that the speaker’s meaning that should be assigned to a legal text is a special version of the sentence meaning of the text, e.g. legal sentence meaning.

The Case for Textualism
Textualism as a normative theory of interpretation gives a clear answer to the question why interpreters of legal texts should aim for interpretations that yield that “plain meaning of the text.” The answer to this question is that plain meaning best serves the rule of law values of limited powers, predictability, certainty, and stability. One of the important rule of law values is that law should be public: law should be accessible to ordinary citizens. Ordinary citizens are likely to interpret statutes to have their plain meaning, because ordinary folks rarely have the training to understand legislative history and even if they did have such training, it would simply be too costly to analyze the legislative history of statutes to determine their meaning.

The same difficulties that afflict lawyers are plague judges. Moreover, most legal research done for federal judges is done by very young lawyers serving as law clerks. This group usually lacks experience in researching legislative history, and their performance is likely to be highly variable. Moreover, because legislative history will frequently contain many conflicting, ambiguous, and vague statements, it is possible that legislative history is easily subject to manipulation, giving judges the opportunity to support their own policy preferences with evidence of the “intentions of the legislature.”

Textualism as a Practical Methodology
Even if textualism does not provide the best ideal theory of legal interpretation, it still may provide the best practical method of interpretation. Suppose that we want to interpret statutes to achieve the purposes of the legislature. At first blush, it might seem that the best way to do this would be to have courts and officials employ an intentionalist methodology, combing the legislative record for evidence of legislative intent. But as a practical matter, it may turn out that judges aren’t very good at doing this. So it might be the case that real-world judges are more likely to implement legislative purposes by employing a fairly simple plain-meaning approach as methodology for statutory interpretation. And to the extent that legislators know that judges will employ this approach, legislators could draft with the expectation that judges will use a plain-meaning methodology, which presumably would lead to even closer fit between the plain meaning and the achievement of legislative purpose.