A "Geometry Net" is a flattened out three dimensional solid (a three dimensional shape) -- like a cube, a prism or a pyramid. When you cut out the "net", fold it and glue it together you can see what the three dimensional shape looks like.
A three dimensional shape is called three dimensional (or 3D) because it has three dimensions: length, width and height.
The two most common measurements we need for a three dimensional shape is:
Creating paper models like these geometry nets allows children to explore the more familiar 2 dimensional shapes that go into making a three dimensional one.
I think it's particularly fun to do these during Christmas -- you can hang the three dimensional shapes on the Christmas tree.