This is the "Saint John the Baptist Preaching" by Raphael. Painted in 1505, it's a predella panel from the Ansidei Altarpiece for the chapel in S. Fiorenzo, Perugia. This altarpiece was a major work of Raphael's and it's believed that he completed at least five predellas, like this one.
In this painting, John the Baptist appears as the forerunner of Christ, standing on a hillock preaching to a crowd of men from Luke 3: 1-17. He can be identified by his traditional camel-skin costume and the thin reed cross in his left hand. Raphael uses a wide range of colours and lively rhythms to convey the varying reactions of the crowd.
Notice how each figure is distinct, with different costumes and expressions. The man in purple listens attentively, while the fat man in yellow shows scepticism. There are even two babies on the rock, recalling the infant Christ and John the Baptist from other images by Raphael.
The predella was placed beneath the main panel of the altarpiece, which is actually housed here at the National Gallery, under a different title, "The Ansidei Madonna". When complete, this scene would have been pointing to the identically dressed Baptist in the main panel above.
Saint John the Baptist Preaching 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.