The surcoat was, like the jupon that replaced it, an outer garment made of rich material. It was worn over armour (at this point little more than chain mail) and was usually decorated with a coat of arms. It could be very long and it is thought that, in the thirteenth century, some were padded. As you can see from the picture above, it was basically a sleeveless tunic.
Those who study these things use effigies and contemporary pictures to try to work out what medieval knights were wearing. Since the surcoat typically covered the knight from neck to well below the knee, it is very difficult to work out what was worn underneath.
It’s probable that the surcoat was copied by Crusaders from the Saracens in the twelfth century in order to keep the heat and dirt from their armour. It was also used for identifying its wearer in tournaments, which had become popular a few decades earlier, and in battle. The introduction of face-covering helmets made identification in any circumstances difficult and a surcoat that stayed on the knight was far more useful than a decorated shield which could be dropped in a battle or not even used. In the twelfth century a tournament was little more than a mini battle, so surcoats were very useful. Surcoats were still being used to identify the dead at the Battle of Crécy in 1346, although the practice of war had already turned against them.
It was eventually replaced by the much shorter and padded jupon, which was more practical for a man fighting on foot. Any English knights still wearing them at Crécy must surely have cut their surcoats short, since they were mainly on foot that day and wouldn’t want a long tunic flapping about their legs.
Sources:
A Dictionary of Medieval Terms and Phrases – Christopher Corèdon and Ann Williams
Knight – Robert Jones
The Tournament in England 1100 – 1400 – Juliet Barker
Tournaments – Richard Barber and Juliet Barker
A Writer's Perspective 