Wikipedia:

In literature and drama, the term everyman has come to mean an ordinary individual, with whom the audience or reader is supposed to be able to identify easily, and who is often placed in extraordinary circumstances. The name derives from a 16th century English morality play called Everyman.

The contemporary everyman differs greatly from his (or her) medieval counterpart in many respects. While the medieval everyman was devoid of definite marks of individuality to create a universality in the moral message of the play, the contemporary storyteller may use an everyman for amoral or, to some ways of thinking, immoral purposes.

And this is a really great short synopsis on a theatre history site, along with texts for the play. {See also: Common Man, and Philip K. Dick’s Mercer Box}