This is the 15642nd most frequent Russian word.
прикрыв
"Прикрыв" means "having covered" or "having shielded" in English.
The word 'прикрыв' indicates the action of covering and is used in a participle form describing how he hid.
Он спрятался за деревом, прикрыв лицо руками.
He hid behind the tree, covering his face with his hands.
Here, 'прикрыв' is used to portray the action of shielding, specifying her condition upon entering.
Она вошла в комнату, прикрыв глаза от яркого света.
She entered the room, shielding her eyes from the bright light.
In this sentence, 'прикрыв' describes the action of covering, indicating the boy's position as he sat.