Makeup in poker refers to the amount a player playing on a stake has to recover before making a profit on the original investment. This concept usually applies to long-term backing deals.
Say a player receives $1,000 from their backer to play in ten $100 tournaments. They bust out from the first eight they play, spending $800 for the buy-ins. This means that the player now is $800 in makeup.
Finally, in the ninth tournament, the player makes the final table and finishes in 4th place for $5,000. From that money, they first need to cover the makeup, which is $800. Then, they deduct the original buy-in from the tournament they cashed in ($100), bringing the total profit for the running stake to $4,100, which can be split between player and backer in agreed terms.
In long-term backing arrangements, makeup is very important for the person doing the staking, and it is almost always paid first before doing any kind of profit split on a stake.