Ace high refers to a hand in which the highest-ranking card is an ace, and there’s no pair or any other higher valued combination of cards possible.
The ace high poker slang is pretty common among players, both pros and casual ones.
For example, let’s say that you have an 8-A hand. The community board runs out 3, 4, 5, 10, and K, with not enough matching suites to give you a flush. In this case, the only value you have is that you have the ace high hand.
To win with such a hand, you have to be very fortunate, as the only way to take the pot is if all other opponents have lower overcards and no pair.