Textuality refers to the "text"-ness of a given object, so to understand "textuality," you have to understand what critics mean when they talk about "texts." Simply put, a "text" is any object that contains meaning and can be "read" or interpreted. A book or poem is a text, of course, but so too are movies, symphonies, advertisements, a shard of pottery, clothing—almost anything can be thought of as a text, in that almost anything can be interpreted and have meaning drawn out of it. In this sense, being a text has less to do with what a thing is than with how humans interact with it.
Textuality, then, is a kind of shorthand way of referring to the all the qualities that go into making a text and a way of referring to the potential for a text to be interpreted. This has to do with the physical qualities of the object (what it is), the context of the object (where you found it), its relationship to other "texts", and its "readability" and the various "meanings" such interpretation is able to draw out of the text.