There are monsters out there. Most people don't believe in them, but they're real. Mostly, when someone finds out that monsters are real, that's just before they die.
But some people are mean enough, smart enough, crazy enough, or hurt enough, that they live. And some of those people go and hunt down more.
That's who you are – someone who decided to go on a crusade against the evil critters that are scurrying around out there.
It could be that you make a stand and defend your hometown from everything evil that comes there. It could be you take to the road and go find them. It could be you have magic powers that help out. It could be that your name came up in prophecies thousands of years ago.
The one sure thing is you aren't gonna go back to your old, safe life.
You can read the hunter playbooks
if you want to see a little more.