This is the "Philip IV hunting Wild Boar (La Tela Real)" by Diego Velázquez. King Philip IV of Spain and his entourage are in the midst of a boar hunt, an expensive activity traditionally staged to mark special occasions.
You can see the King on horseback, just right of centre, pointing his weapon at a charging boar. He's accompanied by his first minister, the Count-Duke of Olivares, and possibly his two brothers, Carlos and Ferdinand, who are all armed with U-shaped forks called horquillas.
On the other side of the enclosure, a pack of dogs is attacking another boar. Outside it, three horsemen ride across the hillside, requiring skill to navigate this terrain. The Queen, Isabella of Bourbon, and her companions watch from the safety of their carriages at the edge of the arena.
In the foreground, you'll see a crowd enjoying the spectacle of the hunt, but many are distracted - a young man is drinking from a jug, three noblemen are chatting, and a man is loading up a mule. Velázquez gives prominence to these secondary figures rather than the King or main action.
This painting was probably commissioned for the King's hunting lodge at Torre de la Parada on the outskirts of Madrid and is one of Velázquez's largest landscapes, measuring nearly two metres high and three metres wide.
Philip IV hunting Wild Boar (La Tela Real) 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.