Supposedly he is based on Ragnar Logbrock, a ruler mentioned in several works of Old Norse poetry and Sagas. You can think of him as sort of a Viking King Aurthur figure.
I won't go into any of his supposed exploits, so as to not potentially "spoil" the future show for you. However, if you want you can read up on it on the wiki page I linked. The original sources are:
- the Anglo-Saxon Chronicle (9th-century),
- book IX of the Gesta Danorum (12th-century),
- the Tale of Ragnar's sons (Ragnarssona þáttr), a legendary saga,
- the Tale of Ragnar Lodbrok, a sequel to the Völsunga saga,
- the Ragnarsdrápa, (probably 9th-century),
- the Krákumál, Ragnar's death-song (12th-century).
The English king depicted in the first season is supposedly Ælla of Northumbria, who appears in one of the aforementioned legendary sagas. While the timing isn't too far off, Alfred was king of Wessex, not Northumbria.