Frost heave is when the ground under your shed rises and falls due to the moisture in the ground freezing and thawing. This is not a major problem if your shed is not attached to any other structure and if free to move on its own. You can fix excessive out of level simply by re leveling your shed.
The permanent fix to frost heave is to build footers that extend down below the frost line and attach your shed to these. Frost heave is not a problem in warmer climates.