。
When she saw me, she waved.
Literal
She [topic-は] me [object-を] sees [when-と] hand [object-を] waved.
The conjunction と after a dictionary-form verb works as a temporal/conditional 'when X, Y' — most commonly used for events that follow naturally or automatically from the trigger. Distinct from ~たら (more general 'if/when'), ~ば (formal/conditional), and ~なら (contextual 'if X is the case'). The と+plain form combination feels especially common in narrative descriptions of habitual or sequential events. 手を振る ('to wave one's hand,' literally 'shake the hand') is the standard collocation for waving as a greeting or farewell.