This is a ‘tronie’, or a study of a certain type or character – in this case a shepherd. The man is wrapped in sheepskin and wears a wreath of ivy leaves on his head. He seems to pause his flute playing in order to look at us over his shoulder.
Backer himself may have been the model for the shepherd. He painted very rapidly and let the ochre base show through in places, like in the grey background, for example.