This is the 12891st most frequent Russian word.
легли
"Laid down" (past tense, plural).
The word 'легли' is used here to indicate the action of lying down in the context of preparing to sleep.
Мы легли спать рано вечером.
We went to bed early in the evening.
The word 'легли' signifies the action of objects positioning themselves flat or settled.
Книги легли ровно одна на одну.
The books lay evenly one on top of the other.
The word 'легли' is used metaphorically to describe clouds spreading out or settle like a layer.