Reference through eNotes gives us a definition:
Textuality is a concept in linguistics and literary theory that refers to the attributes that distinguish the text (a technical term indicating any communicative content under analysis) as an object of study in those fields.
The term textuality comes from linguistics, the study of language and communication. One way that humans communicate is through "texts." You can think of the word text as being roughly equatable with "message." Texts come in many forms--books, magazines, e-mails, eNotes posts, music, sign language... you name it! When you refer to these different forms of texts, you're referring to their textuality. In other words, textuality explains what makes a book a book, rather than a magazine.
The idea of textuality comes along with the assumption that the form in which a message comes to you will influence your interpretation of the message. The textuality of a richly illustrated hardcover book is different than that of an e-book file that you download onto your phone. The words are the same, but the textuality is different. For many people, different textualities also mean different reading experiences. We may encounter this idea more and more as online/digital reading becomes more prevalent and hardcopy materials have to fight for survival.