To earn the GM title, a player must reach a FIDE rating of at least 2500 at some point and achieve three "norms" — strong tournament performances (roughly GM-level results) against sufficiently strong and diverse opposition, typically including title holders and players from multiple federations, within a specific set of tournament rules.
Because both the rating threshold and the norm requirements are demanding, only a small fraction of serious tournament players ever earn the title — a few thousand people hold it worldwide at any given time, compared to hundreds of millions of active players. Separate Woman Grandmaster (WGM) and correspondence/composition grandmaster titles also exist under FIDE's rules.
The GM title, once awarded, is held for life regardless of a player's rating afterward. It remains the standard shorthand for "elite chess player" in commentary and media, even though playing strength among grandmasters themselves varies enormously, from just above 2500 to over 2800.