Wikipedia defines it thus:
The
Christ myth theory (also known as the
Jesus myth theory,
Jesus mythicism or simply
mythicism) is the proposition that
Jesus of Nazareth never existed, or if he did, he had virtually nothing to do with the founding of
Christianity and the accounts in the
gospels.
[1] Many proponents use a
three-fold argument first developed in the 19th century that the
New Testament has no historical value, there are no non-Christian references to Jesus Christ from the first century, and that Christianity had pagan and/or mythical roots.
[2]
Christ myth theory - Wikipedia, the free encyclopedia
So I guess there's two ways of looking at it: Jesus didn't exist or Jesus, or a person at least, serves as a root for the stories but is basically buried under mythology that his later followers attached to him. Either way, the Jesus of the Bible didn't exist, is what it is saying.