A bard, during the Anglo-Saxon times, was an important person when it came to the transmitting of stories. They were a large part of the entertainment which was expected in the castles of the nobility. The bard's job was that of a poet--poetry typically sung in order to tell of the accomplishments of his employers ancestry or of the employer himself. Bards were typically found in both the Gaelic and British societies and can be compared to the English scop of the times.
Simplistically, the role of the bard was to tell the tales of the cultures/societies folklore and actual history. The role of the bard was important given this method was how stories of history were transmitted to other generations.
For example, the great tale of Beowulf (in its time) was told by a bard or scop in order to keep the tale alive and entertain during great celebrations.