This is an incentive payout for new players. Major casinos offer accruals up to 100% of the first deposit. There are limits, often 500-1000 dollars (euro). That is, for example, if a player replenishes the game account with 200 conventional units, the casino will credit the same amount as a bonus.
Sometimes instead of the term "welcome" the word "deposit" is used. This does not change the essence of things: a bonus in combination with one or another option means the same thing.