This is the formula used by Slush's pool to determine how much you would receive if a block is found:
(25 BTC + block fees - 2% fee) * (shares found by user's workers) / (total shares in current round)
The estimated reward, as previously explained by jonnybravo0311, is what you would receive based on that formula if a block were to be found at any particular moment. As more valid shares are sent by you and other miners to the pool, every miner's percentage of contribution changes, thereby affecting your current estimated reward.
All of this is explained in the main page of the pool. It is highly recommended that you read a document before signing it.