this challenge is where you land a base on the mun using only stock parts, I've always wondered why you can't build and launch a spacecraft from another planet or moon. the challenge is to launch a base (several launches allowed) to the mun or minmus that has all the basic parts of a rocket and a way to assemble them however you like, then build a rocket on the kerbin moon of your choice, then launch and retrieve it on kerbin. even better, make several rockets and do the same thing.
good luck