You are very familiar with gravity. It is the the force that keeps you from flying off into space when you leap off the ground. It is the force that pulls you down on the scale when you weigh yourself. Since different planets have different diameters and different masses, the amount of gravity at each planet's surface is different. The actual relationship is pretty interesting.
Greater mass means greater surface gravity.
But, greater diameter means lesser surface gravity. Let's look at the surface gravity for the planets. Again, we'll base all of this an the familiar surface gravity of the Earth. If you were standing on the surface of Mars right now, you would find that the surface gravity there is less than it is here on the Earth. That means you would weigh less on Mars than you do on Earth.