since, as the other fellow pointed out, this question has already been asked and answered and resides in the archives/index of he site, this is pretty much free points. So thanks for that.
The starwars program was a space based missle defense system. It was to rely on satellites and orbital lasers to destroy enemy missles before they could harm the united states. Reagan attempted to make it because of the cold war and because with the pace technology was advancing it seemed possible at the time that by completion the appropriate technologies would exist. This did not come to pass (so far as I know). The reason for a missle defense is that it allows you to annihilate those without them. If you have one and somebody else doesn't, you can strike them while they can't strike you. It also makes terrorists less of a threat since they can't highjack missles and use them effectively in against a sufficient missle defense. The general fear of the soviet union helped make congress more agreeable to the defense spending on that project.