Once the belt is at the desired tightness, the prong will go through one of the holes on the strap, and hold the belt in that position.
Some belts may also have what is known as an end tip. An end tip is usually made of metal, and protects the end of the strap that is opposite the buckle.
The end tip can be greatly beneficial, as when the belt comes on and off, the end of the strap that goes through the buckle can become frayed from friction with the buckle, loop, or parts of the wearer's clothing.
So, in sum, that's
- Buckle (frame, prong)
- End tip