This is the only picture known to be in existence of John Fizalan, Earl of Arundel, 1408-1435. He was a renowned jouster, and is portrayed here bearing the Fitzalan gules lion rampant d'or on his surcoat and horse’s caparison. 

It was crucial for the knight’s coat of arms and heraldic colours to be prominently displayed, so he could be easily recognised in a tournament.

The surcoat also served to cover a chainmail hauberk or plate armour from the hot sun, protecting the knight from heat exhaustion. The caparison was part of a horse’s armour, and was padded to give some protection against arrows. For a horse, a joust was the same as going into battle, with roaring crowds, blaring trumpets and clashing weapons.  The animal wore heavy leather or iron barding to protect its head and carried a knight weighing somewhere between 200–300 pounds in full armour, so itself faced serious risk of exhaustion, besides being injured or trampled.

The picture can be found in the Harley Manuscripts, a collection of over 7,000 medieval manuscripts assembled by Robert Harley (1st Earl of Oxford) and his son Edward Harley in the late 17th and early 18th centuries, now housed in the British Library.
(BL Harley 4205, ff. 36v-37)

His opponent is probably a Mortimer or other Marcher noble family member, wearing azure and gold.