Exactly what it sounds like. It's the small peg at the front, and the small groove at the rear of the barrel that you line up on the target to get the bullet to hit home.
Iron sights are the most common an are sometimes interchangeable.
The sights.
Aiming.
No.
the process of lining up a firearm's rear and front sights
Check w/ Browning, but I think you are correct- BOSS will interfere with iron sights.
Look through your iron sights, or plastic sights, or whatever.
There are several. You can attach laser sights, silencers, flashlights.
most of them were iron sights, very primitive v notch or battle sights
Wolfenstein 3D
No. Iron sights or peep sights (aka aperature sights) are the only type allowed for the merit badge.
In AIT you'll probably only use iron sights, which you need to get the hang of before you move to reflex sights and optics.