On Earth we are kept down on the ground by a downward force due to the Earth's mass; we call it the force of gravity. When astronauts lift off in a rocket, they experience extra downward forces because of the rocket's high upward acceleration. We call the forces they experience g-forces, g standing for gravity. The astronauts experience similar forces during re-entry, when their spacecraft is braked hard as it slams into the atmosphere.
In the early days of space flight, astronauts had to endure g- forces up to 10g - ten times the normal pull of gravity. Under such forces they could scarcely move. Using modern launch vehicles, g-forces are kept to a minimum. Shuttle astronauts, for example, experience little more than 3g, which is similar to the g-forces anyone can experience on some of the latest roller-coaster rides at fun parks. The mild g-forces experienced on the shuttle is one reason why ordinary people, and not just superfit career astronauts, are able to fly on the shuttle with the minimum of training.
