Short answer: Winning the league + domestic cup + Champions League in one season. Only 8 clubs have done it.
The Treble is football's holy grail. Famous Trebles: Manchester United 1999 (Sir Alex), Barcelona 2009 (Pep), Inter Milan 2010 (Mourinho), Bayern Munich 2013 + 2020, Manchester City 2023. Only 8 clubs in history. See full Treble explainer.