Definition of overcard
An "overcard" (could be translated as "map above" or "higher map") is designated in relation to a table. If our hidden hand is 8-9 and the flop is 2-2-5, we have 2 overcards to flop. Designating our overcards tells us that we have a outs if a player has a top pair. For example, if you have AK on 2-2-5 and a player has 88, we don't have a pair, but we still have 2 tens to the flop and 2 tens to our opponent's pair. If we hit a king or ace on the turn or river, we will have top pair.
What's the fairness of the overcards?
It depends on where you are in the hand and what hand your opponent is in. But let's say that if you have two overcards (AK) on a flop 2-2-9 and your opponent has a pair of 7's, your two overcards will give you a 25% equity on the flop, which is marginal. When you consider playing your hand aggressively, simply having tens is not enough. But if you have a flush draw or straight draw to go with those overcards, you'd be in more trouble already!