They're going to build a space station near the moon and possibly also a base on the surface in order to establish a permanent human presence there. The two main purposes aside from pure science are to learn how to live in deep space and on other planets, and to figure out how to mine the vast amounts of frozen water and minerals on the moon in order to make further space exploration easier. E.g. water has oxygen for breathing and hydrogen which can be used as fuel. It will be very useful for deep space exploration in the future if we don't have to bring it up from Earth.