Makeup is the accumulated losses by a poker player that must be repaid before any profit is split with a staker/backer.
A standard 50/50 deal with makeup means that the staker and player will both ultimately end up with a 50/50 split of profit. Any losses accumulate and are called makeup. This is repaid before any profit is accounted for.
Jon stakes Steve at 50/50+mu. Jon gives Steve $5,000 to play online poker tournaments. On his first day, he loses $2,000. Jon gives a $2,000 reload to Steve so that he is back to the full $5,000 bankroll. Steve now has $2k makeup.
On day 2, Steve loses $4,000. Jon gives him a $4k reload, and now Steve has $6k makeup.
On day 3, Steve loses $3,000. Jon gives a $3k reload, and now Steve has $9k makeup.
On day 4, Steve wins $25,000. Steve pays the $9k in losses (makeup) back to Jon. There is then $11k profit left to split (we’re taking out the $5k working bankroll). Each side gets $5,500. The math looks like this:
$25,000 – $9,000 makeup – $5,000 bankroll = $11,000 to split.
On day 5, Steve continues playing with a $5,000 bankroll and $0 makeup.
This cycle continues until the two sides decide to end the partnership, typically when makeup is cleared and X amount of agreed-upon games have been completed. As you can imagine, keeping accurate records of money sent and received is vital to poker staking.