Most often, Jason is a figment of the imagination in fictional works. There's no concrete evidence to suggest it's based on a true story. It's more about creative storytelling and fantasy.
Well, sort of. There are elements that could be traced back to real stories. For example, the fear of being in a place like Camp Crystal Lake where something bad could happen is similar to real - life fears associated with remote and potentially dangerous areas. But the character of Jason Voorhees himself, with his undead nature and superhuman strength, is pure fiction. It's like the creators took the idea of a menacing presence in a spooky place, which can be related to true stories of killers in the woods or at camps, and then exaggerated it to create a horror icon.
Well, sort of. The idea of Jason Voorhees has some roots in real events. There were some tragic incidents at a particular camping area which involved murders. But then, the character as we know him in the movies - the undead, unstoppable killer - is mostly the creation of the filmmakers. They took that kernel of truth about the murders and spun it into the horror icon that is Jason Voorhees.