It's simply a mat for playing LEGO or blocks or puzzles on. I had one when I was a kid and until my kids just used a blanket or table cloth. It is much easier to pack up at the end and gives the kids a fun and defined space to play in. I loved these LEGOish colours in this fabric, and I only need 1 square meter of fabric which cost me $5. Then quickly doubled over the edges and sewed a seem with a straight line.
You could make it as fun as you want with applique pictures or attaching words like I did or you could add a drawstring to convert this mat into a bag at the end.
LEGO just got more fun.