A poker ante up simply refers to the players putting their ante in the pot. An “ante” is the amount of money everyone has to pay before the start of each hand.
For example, you might find this if you’re playing in a tournament with $100/$200 blinds and the ante is $50.
This means that, prior to every hand each player has to ante up $50 for their hand. In such cases, the dealer might say: “ante up!” to signal this.