Materiality is the quality of a physical substance that you can actually feel. Seeing a fluffy beanbag chair online is one thing, but in-person, you can appreciate its materiality.
Materiality is a complicated word with several definitions. One way to think of it is as "the quality of consisting of matter." Architects talk about the materiality of actual buildings constructed with materials (rather than images rendered on a computer). When anthropologists discuss the materiality of artifacts, they're talking about their physical properties. In law, on the other hand, materiality means "relevance" or "importance" — in other words, how much something matters.