Compressors have a dB threshold that you set. Once the volume hits that dB, it "folds" the volume. The "folding" is usually expressed as "ratio"... Think of it as a guy next to your mixer that watches the UV meter and once it hits a level he will turn the volume knob down by a % (50% for half way) or whatever the ratio is measured in on the plugin. Once the volume drops below the db threshold, he pushes the volume back up.

The compressor will have an attack and release. That's just how quickly the guy applies the volume down and how slowly he turns it back up.

As you can imagine a compressed wave form is a lot less peaky... The volume is squashed down and often softer (in terms of loudness).

That's why you have an input gain and output gain. Input pushes the dB higher before compression and the makeup/output gain pushes the total volume of the squashed file back up to expected loudness.

Using slow/fast attacks and releases gives you pumping sounds on drums, and makeup gain pushes other overlooked sounds into the foreground.

A multi and compressor applies this per frequency spectrum. So you have threshold/attack/release/makeup gain for bass, mids and top end.

This is all general terms and some compressors may work slightly different. (Or a lot).

I'm not personally familiar with OTT plugin, but I assume it's a easy multi band compressor of sort.

Compressors are probably the most powerful tool in any audio setup, but is easy to stuff up if you don't know what you're doing.