It refers to a time when you were inexperienced and young but also very idealistic and passionate. The hardcore-punk band Minor Threat released the song called "Salad Days" One of my favorite bands, have a listen: https://www.youtube.com/watch?v=86PVYhAhBGY

Salad days= time period of great success or maximum prosperity. (This is its current use, especially in the US, although per wiki, the original meaning was "period of youth, optimism, etc."). Example: During my salad says, I drove a BMW.