This man looks at ease: his jacket open, his collar loosened and his hat tilted back. His pose is unusual, as people normally looked their best when sitting for a portrait. This man may have been an acquaintance of Rembrandt, and the portrait was probably not a formal commission.
The portrait is very virtuoso, and Rembrandt knew exactly what he was doing. In some places he used his palette knife, and elsewhere he scratched in the wet paint with the handle of his brush. This is how he suggested the springy grey hairs under the edge of the hat.