It's deceptively simple: eight satin-brocade covered rectangular and square cardboard boxes backed into a square tin: small, light, noiseless, and unbreakable, if not indestructible. (The boxes were sold as a set on Montessori Services. I found the tin on my own.) But for whatever reason, toddlers find them fascinating.
The Montessori exercise, which my two-year-old is performing here, is to take the lids off the boxes, and then try to put the correct lids on each box, matching both the shape and the color of the box. She then puts them back in the box, which is a little tricky as you have to line up the squares and rectangles to get them to fit.