Specifically, "what is it?" is a weird question. People have been answering it in increasing detail for a long time and it still frustrates us. First it's elements, then atoms, then a stew of protons and neutrons, which happen to made of sub-atomic particles themselves, until we reach the new "fundamental". Someday, a particularly crafty scientist will be able to solve the quark parity problem or manage to crush one into bits and then we'll have a whole new layer of stuff beneath quarks. Or maybe they won't, but we'll still wonder what a quark is and why God or nature made that the smallest unit, and we'll always want to split it into two.