This is the 13025th most frequent Russian word.
лёг
"лёг" means "lay down" (past masculine form of the verb "лечь").
Here, 'лёг' is used as the past tense, masculine singular form of the verb 'лечь', meaning 'to lay down' in Russian.
Он лёг на кровать.
He lay down on the bed.
In this sentence, 'лёг' is used to question the subject in the past tense about an action of lying down, showing inquiry for clarification.
Кто лёг на диван без разрешения?
Who laid down on the sofa without permission?
'лёг' is demonstrating the subject's action of lying down as part of a sequential narrative in past tense.