What is meant by "Selling one’s soul to the Devil"?

I've become very interested (by it I mean curious) about "selling one's soul to the Devil" in the past few weeks. You probably have heard the most popular examples of this: Robert Johnson, Paganini etc. I researched a lot about this and someone told the method to do it, someone said it can be done in pieces and an answer to a question I found on StackExchange says that it cannot be done at all as we are not the owners of our souls, but God himself. So what does this actually mean? And if somehow somebody is able to do this, what consequences does he have to suffer? How could the myth of Robert Johnson or Paganini be explained? If "selling one's soul to the Devil" is absurd, then why does this concept even exist? PS: I am not Christian (so sorry if I wrote something wrong) but I believe in God and the spiritual war with the Devil.