Baron is a title of nobility. It is the lowest rank in the English nobility, below Viscounts.
It is also the historic generic term for the aristocracy; as in "King John met the barons at Runnymede...." The word "Barons" (in the plural) in British history is the term popularly used to describe the peers of the realm - noblemen - "en mass". Until the mid 14th century they were predominately earls (the English equivalent of the Continental "Count"). The first dukes were created by Edward III for his son Edward the Black Prince (Duke of Cornwall - 1337) and others. Dukes are to this day, the highest honor below the sovereign.
The post-Norman Conquest history of medieval England is largely a story of the tension between the Crown, the Church and the Peerage (the barons). It was not until the late 14th-early 15th centuries that common folk began to play a significant part in the politics of the day as the three protagonists courted the burgeoning merchant classes in Parliament.
Today, a baron is usually a "life peer", created by the sovereign for meritorious service, and with no inheritance rights. It is also a courtesy title.
A baron is also "a very wealthy or powerful businessman" such as an "oil baron."