In the middle ages armor was important to all knights. The very first knights were protected by haubeks that were made out of chain mail. Over the chain mail they wore a surcoat. The surcoat was used to keep the knight from getting too hot in the sun. It also displayed the coat of arms. This type of armor was mostly used in the 1100's.
Then during the 1300's knights started using plate armor. Plate armor was a silvery steel made into different shapes for the different parts of the body. The armor did not cover the neck, back of legs and arms so the wore chain mail in those places. Chain mail was also worn on the feet like shoes.
By the 1400's knights wore complete body plate armor that weighed 40 to 55 pounds. Even with how much it weighed and the fact that it was steel it was actually very flexible.
The armor may have been able to stop a sword blow or a bash from a mace but was no match to a well aimed crossbow bolt or a long bow shaft aimed at the right place. Plate armor was used until the development of rifles and cannons after which the armor was rendered useless.