This picture shows two episodes from the story of Saint George: his defeat of a plague-bearing dragon that had been terrorising a city; and the rescued princess bringing the dragon to heel (with her belt as a leash).
In the sky, a storm is gathering. The eye of the storm lines up with Saint George's lance, suggesting that divine intervention has helped him to victory. Uccello uses the lance to emphasise the angle from which Saint George attacks the dragon, helping to establish a three-dimensional space. The strange patches of grass illustrate Uccello's obsessive concern with linear perspective and his tendency to create decorative pattern.
The story is from a popular collection of saints' lives written in the 13th century, called 'The Golden Legend'. An earlier less dramatic version of the same subject by Uccello is in the Musée Jacquemart-André, Paris.