。
She left her gloves in the car.
Literal
She [topic-は] car [of-の] inside [in-に] gloves [object-を] left-behind.
置き忘れる is a compound verb meaning 'to leave behind' or 'to forget (something somewhere)': 置く (to place) + 忘れる (to forget). 車の中に marks the location where the gloves were left. This verb is more specific than plain 忘れる — it means she physically left the item in a place, not that she forgot about its existence.