The series Vikings that we know and love is based on a Scandinavian tale, The Saga of Ragnar Lodbrok, which follows the adventures of Ragnar and his family, particularly his sons. (Side note of little consequence: Ragnar's name translates to "hairy breeches.")
A saga is a historical account in the broadest sense, but we need to remember that sagas weren't recorded until long after the actual events went down and are filtered through the time in which they are written.
This means, according to Dr. Godlove, "they are based on oral history, passed down by descendants and are more historical fiction than fact, more legendary than accurate depictions of the past of great heroes and warriors."
While the Ragnar of the History Channel's series probably really existed, what we know about him is colored through years of storytelling and retelling and might not be the most historically accurate depiction of actual events. All we heard was "legendary" and "heroes and warriors," so we're mostly OK with this.