Can someone point me in the right direction for edumacating myself about SEF trigger groups?

I found a PTR with one of those in a gun store earlier today and didn't know what the fuck it was.

Didn't ask one of the guys behind the counters because they get mad as shit every other time I've asked them anything so I don't even bother anymore.

My favorite local gun shop closed within this past year and I'm forced to go to these assholes because they're the only ones within a reasonable distance of my house anymore.