A $300 deposit bonus is a promotional offer commonly used by online casinos and sportsbooks to attract new players. When a user makes their first deposit, the platform matches a percentage of that amount, up to $300, effectively boosting the player's initial bankroll. This bonus allows players to explore a wider range of games or betting options without risking too much of their own money. However, it's essential to read the terms and conditions, as wagering requirements and restrictions may apply before any winnings can be withdrawn.