Grind, grinding, or to grind is widely used jargon that describes an action of continuously repeating the same process in order to push yourself to achieve something.
When it comes to casinos, grind refers to a player who keeps wagering small amounts and working their way up to make a profit. While a whale or high roller would bet hundreds or thousands of dollars and hope for a lucky break, a grinder usually relies on their skill and plays it safe.
By understanding the game and betting small, a grinder is not risking a lot, so if they lose, they can still recover and continue working their way up.
Interestingly, one of the well-known professional poker players, Michael Mizrachi, is nicknamed The Grinder because he started his career precisely by grinding. It clearly paid off as he currently has 5 WSOP bracelets under his belt and numerous achievements in prestigious tournaments.