When she was writing the book that was to become The Fountainhead, Rand used a different title: Second-Hand Lives. This idea of living a second-hand (or conventional) life crops up near the end of the book in a scene with Roark, the protagonist, and his frenemy Gail Wynand. Neither Roark nor Rand was a fan on the idea of buying a life at a consignment store.
The shift to a new title is a pretty significant one: Second-Hand Lives sounds fairly negative. It sounds like it's going to focus on the sad little lives of people who aren't really original. Rand couldn't care less about sad little lives: she wanted to focus on one man living in what was, according to ol' Ayn, the perfect life.
A fountainhead refers to an original source, like a refreshing spring-fed lake. But what exactly is this titular fountainhead? Well, our protagonist Howard Roark lives an original life of own choosing, inspired by the idea of Individualism. Both Roarkypoo and Rand's darling Individualism are the fountainhead in The Fountainhead.