Geometry is the posh name for any topic involving shapes. I've assumed you already know the names for common 2D shapes but if you need a refresher I recommend you look here, focusing in particular on the triangles and quadrilaterals. The topics I've chosen here range from the basics (Area, Perimeter and angles) to the more advanced Pythagoras' Theorem. Although this isn't a fundamental mathematical topic I find it's one that people can pick up quickly, as long as they have a good understanding of square numbers and square roots. Have fun!