Isaac Newton was a great scientist and mathematician who lived more than 300 years ago. He understood and wrote about many of the laws of motion that we see at work every day. To explain how one body can orbit another, he asked his readers to imagine a cannon on top of a very, VERY tall mountain. (Our cannon is on a very tall, imaginary lifter that goes up and down, but it is the same idea.)
The cannon is loaded with gunpowder and fired. The cannonball follows a curve, falling faster and faster as a result of Earth's gravity, and hits the Earth at some distance away.
What if we use more gunpowder? Here's what might happen: (Note that these amounts of gunpowder are just imaginary, not meant to be precise! Also, we are ignoring the fact that the air would cause drag on the cannonball and slow it down.)
Amount of Gunpowder
2 bags of gunpowder:
Cannonball goes faster and gets farther before gravity pulls it back to Earth.
3 bags of gunpowder:
Cannonball is going so fast that it falls all the way around the world. It is in orbit!
4 bags of gunpowder:
Cannonball orbits Earth again, but goes even higher at the peak of its arc.
5 bags of gunpowder:
Cannonball is going so fast it completely escapes Earth's gravity and heads out into space, maybe to an asteroid or Mars or Jupiter!
The same thing happens when the Space Shuttle or a satellite is launched into orbit. The rocket boosts the spacecraft up to the height of a "very tall mountain" and also gives the spacecraft its forward speed, like the gunpowder gives the cannonball. So the spacecraft just falls all the way around the Earth, never hitting the surface. The curve of the spacecraft's path is about the same as the curve of Earth's surface.
So astronauts orbiting Earth aren't really weightless, they are just falling . . . and falling, and falling!