。
She tripped on a stone.
Literal
She [topic-は] stone [on-に] tripped.
つまずく means 'to trip' or 'to stumble' — the に marks what she tripped on. A simple, concrete sentence. つまずく is also used figuratively: 人生でつまずく ('stumble in life'), 問題につまずく ('get stuck on a problem').