'Spill the tea' means 'share the gossip' or 'tell me the truth'. 'T' originally stood for truth in 1990s drag and ballroom culture; the spelling 'tea' became standard online in the 2010s. 'Come on, spill the tea' = 'come on, tell me what happened'. If your teen says it, they are asking for gossip or information. Harmless; affectionate among friends. The noun 'tea' on its own ('what's the tea?') has the same meaning.