It would tell us they didnt accidentally stand on a Lego brick barefoot, and interpret it as a primative act of war ;)

Seriously though, if they build the human figure it would show if they understood our basic concepts/instructions, but not necessarily that is how we look. If they built a figure in their own image it would show they understood the instructions was how to build our form, and that they wanted to communicate with us by sending back a model of their own form.