How does the pill work and when is it safe for me to have unprotected sex?

I’ve been on the pill for over a year now but I’m still so confused as to how it works in terms of when it’s safe for unprotected sex?? My boyfriend and I have been together for almost 5 years and he’s been tested for STIs etc so that’s not something I’ve had to be worried about but I’m only 17 and not ready for a child yet. After a pregnancy scare early this year I want to be sure of things before we do anything without other means of protection.