When the dealer has an Ace up and you have a blackjack, the dealer will offer you "Even Money" before checking for a dealer blackjack.
A basic strategy player should always decline the offer of even money, and hold out for the full 3:2 payout, even though sometimes they'll push and collect nothing instead.
"Even Money" is just a shortcut for taking insurance, which is a bad bet for the player. A longer explanation is available near the end of the
Blackjack Rules page here.