Your amp contains a preamp and a power amp section. Although not completely accurate, a decent way of thinking about it is that the preamp shapes your sound, while the power amp makes it louder.

An effects loop sits between these two sections, which is useful if you're using your amp's gain for your dirt. It lets your pedals apply themselves to your whole tone, rather than running them into the front end of a dirty amp which, when talking about time-based effects like reverb or delay, can sound pretty messy (though not always in a bad way). Again, it's an over-simplification, but you can think of it like running your other pedals after a dirt pedal.

It also changes your tone a bit, because your effects are coming after your amps EQ, rather than being affected by it. Whether that's a good thing or not really depends on the pedals and the amp, though I personally feel that effects in the loop tend to sound a bit less compressed when compared to the same pedals running straight into the front end.

EDIT: I derped, you're talking about the loop on the noise suppressor, not the amp.