Symmetry is a very important concept that is useful in all of the sciences. There are four basic symmetry elements - axis of proper rotation, axis of improper rotation, mirror plane, and point of inversion (technically these last two are only special cases of improper rotation, so you could correctly say there are only two symmetry elements). The most readily understood element is the mirror plane, where you can cut an object in half, and every point on one side of the mirror plane reflect across to an identical point on the other side. You're surrounded by objects with bilateral symmetry - a coffee cup, for instance, or the chair you're sitting on. Even the human body has rough bilateral symmetry if you ignore details like parting your hair to one side or wearing a watch on one arm and not the other. Here are a couple of LEGO objects with mirror planes, Pete Reid's LL-497 (if you ignore the Classic-Space logo) and Luis Baixinho's Butterfly.*
*Just to be pedantic, and before someone calls me on it, the fact that the word LEGO is written on the studs means than no LEGO object actually has bilateral symmetry, because you would have to have the mirror image word written on the other side of the creation.