In addition to Lucio's explanation, gravitational potential energy depends on height.
When you hear "potential energy" there is energy that is stored that can be released. For the case of any mass elevated by h (height above ground), it has the potential to fall down. The greater h, the greater amount it could fall and thus greater gravitational potential energy. For problems that generally do not involve extreme heights, it is OK to use
(Verified by the definition of work:
Gravity is an accelerative pull towards the center of the earth. This means that the closer to the center of the earth you are, the more you are affected by the pull.
This is the formula used to calculate gravitational force between two bodies. F is the force of gravity, G is the constant (they give you that number below) and r is the distance from the center of the earth. If M1 is the mass of the earth and M2 is your mass, you will notice that the farther you get away from the earth (the larger the r quantity) the smaller of a number F will turn out to be. This means that a larger distance from the center of the earth will make the force on you smaller.