Definition of overcard
An "overcard" is a card on a board. If our hole hand is 8-9 and the flop is 2-2-5, we have 2 overcards on the flop. Designating our overcards tells us that we have outs if a player has top pair. For example, if you have AK on 2-2-5 and a player has 88, we have no pair, but we still have 2 overcards on the flop and 2 overcards to our opponent's pair. If we hit a king or ace on the turn or river, we have top pair.
How fair are the overcards?
It all depends on where you are in the hand and what your opponent's hand is. But let's say you have two overcards (AKs) on a 2-2-9 flop and your opponent has a pair of 7s, your two overcards will give you 25% equity on the flop, which is marginal. When you consider playing your hand aggressively, just having overcards is not enough. But if you have a flush draw or a straight draw to go with those overcards, you're already in business!