What does "fast life" mean?

Plays hard.  doing all those fun risky things that shorten your life.  Likely to die young.

It depends on context, but generally "doing a lot of things", typically for excitement.  The excitement is what makes it "fast".  Its opposite would be boring, which is "slow".  The context will usually say how the life was "fast", what was being done to make that excitement.  Gambling, carousing, traveling the world non-stop, or what?

A person that spends a lot of money quickly. And essentially doesn't think of the future. "Wine, women and song". If that puts an image in your mind.