What is a ladder reset?
So a ladder reset means that your current ladder rank, league rank, and point values will be reset. Its important to note that your hidden rank is not reset.
Ladder Resets are designed to occur periodically, but what that period is has changed a couple times. The current prevailing belief (after Blizzcon) is quarterly.
While your Hidden Rank is designed to measure your skill accurately, your ladder rank doesn't do this very well. A reset is designed to help correct this disparity to some degree. Because your hidden rank isn't reset the system will still be matching you against equal quality players and using that to determine your League.