Answer:
Gravity is a force that makes every two bodies in the universe attracted to each other. Your computer and you, the earth and you, etc... The more massive two objects are, and the closer they are, the more attracted they are to each other.
When we're on the earth we have a feeling of been pulled down because we are attracted to the earth, and the earth is equally attracted to us, just the earth is so massive that it does not move very much towards us. So we always are being pulled towards the center of the earth, our down.
In space though things are not necessarily pulled in any one direction all of time. There is no down. And it is because there is no real down in space that objects, to us earthlings, appear to float. The planets are being pulled by the gravity of every single object in the universe, but there is no main object, like earth to us, that makes everything in the universe fall down, and so everything just floats were it is.