The term overcards is used when talking about community cards, more precisely, to say how a community card ranks higher than the ones you have in your pocket pair.
An overcard puts you in a dangerous situation, as someone holding the same-valued card as the overcard automatically makes a stronger hand than the pair in your hand.
For example, you have a strong pocket kings hand. But, when the flop comes, it produces the 8, J, and A community cards. You still have a good pair, but the flop has one overcard to your hand.
Alternatively, some players also use the overcard poker slang to say that they have higher-ranking hole cards than the three community cards on the table.