If you go to the search feature on here you can find some wonderful pictures of I spy quilts.
They are generally "fussy" cut shapes/squares of patterns so you can play the "I spy" game that most of us played as a child.
You find an item on the quilt and say "I spy a ____" and the child has to find it. Some people do 2 like items so they can play a matching game. Others do themes like all travel items, food items, cartoons items, alphabet items, boy themes, girly themes, or mishmash of everything, etc. Up to you as to how you want it to be.
Some people "frame" each shape or square with a sashing fabric to set it apart from the other pieces, while some sew them one after another which does make it harder to find the "I spys". You can make them "age appropriate" also, which gives you a whole other range for older kids. I also have heard where someone was making one for a senior center and they were loving it.
They are for fun and can be for learning. I've even heard/read where some make extra pieces to lay on the quilt so it's like a game board.