A simple tutorial on shape keys. Shape keys allow you to change a mesh without changing the original shape. You can can create shape keys for a character, for example, that allow it to smile, frown, blink, wink, etc.
It could represent an object exploding, a pipe bending, etc. Each key will be given a slider bar, which you can adjust at any time to apply any shape key. The slider allows you apply the shape key from 0% to 100% or anywhere in between.