An all-time high, often abbreviated ATH, is the highest price an asset has ever reached since it began trading. When people say a coin hit a new ATH, they mean its price climbed above any level it had touched before. The opposite, the lowest ever price, is called an all-time low.
The term is popular in crypto discussion because prices can move dramatically, and new highs draw attention. It is a simple historical fact about past price, useful for describing where a price stands relative to its own history.
It is important to remember that an ATH describes the past only. Reaching a new high says nothing about whether the price will continue up or turn down afterward. It is a descriptive term, not a prediction about future movement.