if someone's asking you "What tipped you off?" It's because they want to know how you figured something out, or what clue you got to know about something they were keeping from you, like a secret or surprise.

It can also mean, when used sarcastically, what made something so obvious? (Like if Person A yawns, Person B says 'Are you tired?', Person A could say 'What tipped you off?')

Giving insider information or advise to. "He tipped off the police about the terrorist plot" = " He gave information to the police about a terrorist action."