This is "A Bearded Man", painted in the Italian north around 1527-8. The sitter wears a black pleated silk doublet with pleated sleeves and a soft black cap, possibly made of felted wool, with black knotted braids at the edges. His white chemise has a ruffled collar and cuffs tied at the neck with a black ribbon.
You'll notice that the man holds a folded paper in his right hand - it's not uncommon for portraits from this period to include the sitter's name on a folded paper, as if they were holding a letter they'd just received. However, this particular paper contains no writing to help us identify the subject of the portrait.
The painting was once thought to be by either Pontormo or Lotto, both prominent artists in Florence and Venice during this time. Unfortunately, it's been damaged by extensive cracking over most parts of the canvas, which has made its origins a bit more difficult to pinpoint.
A Bearded Man hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.