A Backer is an investor that supplies a bankroll to a poker player in return for some percentage of the player’s profits. Getting backed (or “staked”) allows players to play at higher stakes than they would on their own bankroll. If the player loses money, the backer is the one who primarily eats the loss.
Example: John is a poker pro who is being backed or staked by an investor for 50% of every tournament that he plays. If John cashes (makes the money) in a tournament, 50% of anything he wins will be paid back to the investor who backed John during the tournament. If John fails to make any money in a tournament, he is usually not responsible to repay the backer for his investment into the tournament.