How you define "free spirit" depends heavily on your definition of "freedom
." But a free spirit is a person who has, values, and lives for their freedom.
For me, the most important types of freedom are intellectual and emotional -- and these are freedoms that are mostly up to you to grant yourself. Which is easier said than done. A free spirit in this sense is not constrained by guilt, closed-mindedness, or unreasonable fear. I'm working on it.
Nietzsche was big on the idea of "free spirits."