A warning was an informal caution by law enforcement given to an individual who was breaking the law.
In 1953, after stopping him briefly, police officers Kevin Mulkahey and Burt Ryan let Benny Russell off with a warning, the latter saying he wouldn't be so lucky next time. (DS9: "Far Beyond the Stars")
In 2022, Toronto City Police let James T. Kirk go with a warning after Sera bluffed that he was a famous civil rights attorney (and therefore more trouble than an arrest would be worth.) (SNW: "Tomorrow and Tomorrow and Tomorrow")
According to Beckett Mariner, Starbase 25 security was crooked and "aren't gonna let us (her and Brad Boimler) off with a warning and a glass of Thai iced tea." (LD: "An Embarrassment Of Dooplers")