。
She tapped the table with her finger.
Literal
She [topic-は] finger [with-で] table [object-を] tap-tap struck.
とんとん is a 擬音語 (sound mimetic) capturing a light, repeated tapping or knocking sound — the kind of sound a finger makes against a hard surface. Same word also describes someone knocking at a door (とんとんたたく). Repeated mimetics like this convey the rhythmic quality. The instrumental で marks 指 as the means: 'with [her] finger.'