at one point in everyone's lives they have tried to use a pogo stick (some more successful then others).
for starters there is elastic potential energy stored in the spring. which is the work that is done stretching, in this case compressing, the spring. you can find out just how much energy is stored through using the equation PEs= 1/2kx^2. where k stands for the spring constant or the stiffness of the spring and x stands for the displacement from equilibrium. (the difference from the original spring and how long/ short it is after you stretch/compress it. also the more you stretch or compress the spring the greater the fore of the spring. in this case it means that the more you compress the spring the higher off the ground the pogo stick will go. have fun pogoing.