Infantry are warriors who fight on foot, as opposed to those who fight from horseback. Infantrymen were common throughout the medieval period, as the accoutrements of the mounted warrior were far too expensive for the average fighter.
Infantry in Period
Infantry dominated the battlefield until about the 11th Century, when the stirrup gained widespread use throughout Europe, allowing mounted warriors to fight more effectively from the saddle. Even after the rise of the mounted noble warrior, infantry were the mainstay of any medieval army, if only in terms of numbers. Most early period armies, such as those of the Norse or Anglo-Saxon, were predomninately infantry. The Normans were the first to introduce the newer, stirrup-using cavalry to battle, which doubtlessly contributed to their victory at the Battle of Hastings in 1066 C.E.
After a lifetime of practice, even knights who would prefer to fight from horseback would have been deadly opponents even on foot. Infantry would not be restored to primacy until the development of accurate and portable firearms.
Infantry in the SCA
All warriors in the SCA, regardless of rank, are infantry, owing to the dangers of fighting from horseback. As such, SCA rules require all combat to be fought on foot. SCA armies therefore resemble early-period armies far more than late-period ones.