TP-in / TP gank:

A TP-in or TP gank is a gank in which one or more heroes teleport to a lane, tower, or allied unit to quickly join a fight and catch an opponent off guard. The term combines the teleport action with the goal of creating an immediate numbers advantage, often to turn a skirmish, defend a teammate, or punish an overextended enemy.

TP ganks matter because they can convert map pressure into a sudden fight and are a common way to respond to dives, side-lane picks, or split-push attempts.

Related: Gank