Gravity (重力 Jūryoku) is a concept addressed frequently in the Dragon Ball universe.
Basically, gravity, or gravitation, is a natural phenomenon by which all things with mass are brought toward (or gravitate toward) one another, including planets, stars and galaxies. Since energy and mass are equivalent, all forms of energy, including light, also cause gravitation and are under the influence of it. On Earth, gravity gives weight to physical objects and causes the ocean tides. The gravitational attraction of the original gaseous matter present in the Universe caused it to begin coalescing, forming stars — and the stars to group together into galaxies — so gravity is responsible for many of the large scale structures in the Universe. Gravity has an infinite range, although its effects become increasingly weaker on farther objects.
Weight training (gravity times mass) is used early in the series as a means of acquiring strength. This begins with the use of weighted clothing, such as the shells worn by Goku and Krillin given to them by Master Roshi. Goku and Piccolo also later drop weights hidden in their clothes prior to battling Raditz.
Later, weight training is made more intense through a different means. Rather than add weight through clothing, heavier gravity environments are utilized. This begins with Goku training with King Kai, whose planet is 10x Earth's normal gravity. The other human Dragon Team later train here while the others are adventuring on Namek. Planet Vegeta and Planet Zoon also have 10 times the Earth's normal gravity.
On his way to Namek, Goku trains in a Capsule Corporation spaceship designed by Dr. Brief. It contained a Gravity Machine to artificially enhance gravity. He trains up to 100x the Earth's normal gravity in it and does various bodyweight exercises.
Vegeta (who would have access due to Bulma being his wife) was seen training with even greater extremes. In the Garlic Jr. Saga he trains with 300x. In the Trunks Saga, Vegeta used to set it to 500x, but was injured as a result. After seeing Vegeta training in it, Yamcha also tries to train in the gravity chamber, with 300x Earth's normal gravity. However, he fails to withstand it. In his battle against Pui Pui, Vegeta describes having trained with 450x, possibly scaled back to avoid being injured as before.
Several years later, in the Baby Saga, while the heroes Goku, Pan, and Trunks are in space looking for the 7 Black Star Dragon Balls, Giru detects the One-Star Dragon Ball in a Deserted Spaceship which is about to fall into a star 70x Earth's normal gravity. After claiming the One-Star Dragon Ball, the heroes attempt to escape it's gravitational pull, get aboard their ship and fly off before time runs out. After trying to use their space scooters to escape, seeing how they cannot withstand the suns force, they decide to fly instead. However, Pan is seen slowly being pulled back, struggling to propel herself forward from the star's gravitational pull.