The reason objects float in the space is that they are in free fall. Normally, things are slowed by air molecules, or dragged towards a planet by gravity. However, space behaves like a vacuum, because there are so few molecules around. You float because the attractive force of gravity in space is much less (because you're far away from massive objects like planets), and you continue to travel in the same direction because there is nothing to get in your way (like air molecules, which would normally slow you down).
Words to Know:
Free fall: the condition of moving freely in an environment in which gravity, and nothing else, is causing acceleration
Vacuum: the absence of all matter, including air
Mass: the measurement for the amount of matter in an object