Myths are things in which people-in-general believe, but that can't be proved to be true.
Generally myths migrate across the landscape between accepted facts and outright fictions, but some take up semi-permanent residence in the territory of myth, where belief is too strong for them to be abandoned, but proof is too arduous for extensive attempt.
In the middle ages, the worship of saints helped expand the myths relating to their lives. The idolism would twist truth unintentionally as well as for political, social or financial gain. A good example is the story from the First Crusade relating to the wood that was mysteriously found in the desert. The claim was that it was from the spear that wounded Jesus Christ on the cross. This was shown to be a good omen and used to justify the push on to "free" Jerusalem.
Some entertainers (eg. the minnesinger, Ulrich von Liechtenstein) also tended to tell tall tales that are hard to prove or disprove due to lack of evidence. As their reputation grew, so did the myths about them.
Some myths have multiple origins but their similarity has melded them together into a single story. For instance, Robin Hood may have been based on real people and folklore and fables on someone called Robin.