A deposit bonus is an additional amount of money offered to participants who deposit their real money into a fantasy football or sports gambling website. Deposit bonuses are a common offer for these types of websites, and are used as a gift to entice players to like that particular fantasy football service and continue to deposit money in the future. Most deposit bonuses are given as "free play" money on the service, and can't actually be withdrawn as real money unless first wagered.
Deposit bonuses can have different values ranging anywhere from a 10% to 100% match on your original deposit. However, they often have a maximum limit on how much of a bonus can be awarded to prevent huge bonuses being given that the company couldn't afford. Deposit bonuses can be a nice gift to build customer loyalty on these services, but can sometimes come with a catch.
On some fantasy sports websites, upon agreeing to receive a deposit bonus they ask you to agree to their terms, which can include additional hidden rules such as minimum amounts you are required to wager or win before you're actually allowed to withdraw money from the service; often times these amounts are higher than the bonus you would receive as a result. To be aware of what bonuses you receive and the rules behind them, make sure to read the fine print on the agreement before accepting a deposit bonus.