The term double up in poker is used by players to describe the situation in which they or their opponents managed to double their chip stack.
For example, say you have 10,000 in chips and pick up pocket kings. You move all in and get called by another player who has more chips than you. They have pocket queens and your hand holds up to win the pot. Since the opponent had you covered, you will have gotten a full double up, getting your stack to 20,000.
The previous example describes what the actual “double-up” is. However, the term is sometimes used even when it’s not technically correct.
In that same example, if your opponent only had 8,000 chips to start the hand, winning the pot wouldn’t quite double you up, but it would still significantly increase your chip count. So, even if it’s not correct in the most technical of terms, players will often say they “doubled up” in this situation, especially if there is some dead money in the pot from blinds and antes.