Gravity is an attractive force between objects. The strength of the attraction depends on how close the objects are to each other, and how big the objects are. So, objects that are close to each other or large attract each other more than objects that are far apart or small.

The reason we don’t float away from the earth is that the earth is both large and close. So, if you jump, the force of gravity pulls you down. However, if you were able to jump incredibly high (or if you were in a rocket ship) you could get far enough from the earth that the attractive force pulling you back would be too weak, and you would continue into space.

The atmosphere doesn’t prevent us from floating away, but it does allow us to live. The atmosphere is gas trapped near the earth by gravity, just like we are. We need the atmosphere to live, because it supplies us the air we breathe, and helps keep the earth at a good temperature for life. Without the atmosphere, not only would we not be able to breathe, but the average temperature of the earth would drop to below the freezing point of water and most life on earth would quickly die.