What is a gravity well?

A gravity well is the pull of gravity that a large body in space exerts. The larger the body (the more mass) the more of a gravity well it has. The Sun has a large (or deep) gravity well. Asteroids and small moons have much shallower gravity wells. Anything on a planet or moon is considered to be at the bottom of the gravity well. Entering space from the surface of a planet or moon means climbing out of the gravity well, something that often takes a huge amount of energy. The larger a planet or moon’s gravity well is, the more energy it takes to achieve escape velocity and blast a ship off of it.