The poker term makeup is closely connected to the practice of backing or staking. It is a number that shows how much a backed player needs to win before they reach the breakeven point.
For example, a player is staked $10,000 to play tournaments. After playing for a month, they are down $5,000, with $5,000 still in their bankroll.
At this point, the player in question is $5,000 in makeup, and they need to win that amount back before they can share any profits.
So, if they were to win a tournament for $8,000, the first $5,000 would go towards makeup to bring the bankroll back to $10,000. The remaining $3,000 can then be split between the backer and the player according to their backing agreement.