The name says it all: It´s a welcome bonus you receive when you make your very first deposit at a casino. The most common type of 1st deposit bonuses is the so called “match bonus” which basically will match your deposit up to a certain percentage. For example; A casino offers you a 100% match bonus up to $200. This means that they will give you 100% in free money on any deposit amount you make up to $200. ie. You deposit $100 and you will receive another $100 in bonus money, making your bankroll $200 or you deposit $150, you will get an extra $150 which will then total up to $300.
Some of these 1st deposit bonus come with extra free spins on top. Sometimes it´s just the free spins you will get when making your very first deposit.