Gravity is a force that pulls something into the center of something. For example, on Earth gravity pulls you towards the core. But when you are comparing the gravity levels to other planets, they might vary because of the size of the planets. For example, when you compare Earth's gravity to Jupiter's, you would weigh more on Jupiter since it is a bigger planet and has higher gravity. Gravity is also what keeps the planets rotating around the Sun. There is no actual gravity in space though. You might be confused by this. Let me explain; gravity is only apparent on planets because as I explained earlier gravity is a force that pulls something into the center of something ,and because the Sun has so much gravity it manages to pull the planets into orbiting itself.
Gravity was found by a guy called Issac Newton who was sitting under an apple tree when all of a sudden an apple fell on his head - then it struck him!