I don't understand constraints, help!

Think of constraints as invisible rods that connect two objects together, one end of the "rod" is at the position of the "Part" object and the other end is at the "Pivot" position. The linear limits defines how much the "rod" can strech in each direction and the angular limits defines how much the "rod" can rotate in each direction. You can also use linear spring and angular spring to make the "rod" into a spring or you may use linear motor and angular motor to move and rotate the "rod".

Note that if the "Base" object has physics enabled the pivot will always be at its position and the "rod" will share the object's rotation.