Back in those days many peasants were tied to a local Lord and had to work for them (a form of slavery).
Some people ran away from these Lords and often the Lord would send out some of his men to look for the runaway "slave".
So these runaway slaves would hide out in the woods, and because the woods were much larger then you could hide many people in the huge woods.
So myths and legends grew up about these mysterious people who lived in the woods, and survived by criminal activities, legends often passed around from town to town via stories and songs.
So Robin Hood was just one of these people who got talked about in stories and songs, but there were many "Robin Hoods" all over the country.