This is "A Man with a Skull" by Michiel Coxcie. Painted in about 1560 or later, it's a portrait of a wealthy gentleman from the region around Brussels and Leuven. The coat of arms in the upper corner suggests he was a member of the Heverlee family. His outfit may look a bit outdated, but his jewelry shows that he had plenty of money.
Take a close look at his hand - it's resting on a skull, which is a reminder to the sitter and us that death is inevitable. This motif was popular in sixteenth-century portraits, like Holbein's famous "The Ambassadors". Notice how the artist has captured the subject's expression, and the way he's dressed. These details help us understand who this man might have been.
Despite its age and importance, this painting had a bit of a complicated history. When it was first acquired by the National Gallery in 1845, it was thought to be a work by Holbein - but later analysis showed that was unlikely. Today, we believe Michiel Coxcie painted it, probably around the time he was working in Brussels as one of its leading portrait painters.
A Man with a Skull 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.