Art silk is like an imitation of real silk that is made from man-made materials instead of natural ones. You know how some toys are fake versions of the real thing? Art silk is similar to that but for silk!
To make art silk, people use fibers made from chemicals instead of the cocoons of silkworms used in real silk. Then they weave or knit these threads together to create fabric that looks and feels almost like silk but costs less to produce.
So basically, when you see a piece of clothing or fabric labeled "art silk," it means that it's not real silk, but it might look and feel pretty similar!