Perceptual loudness (measured by RMS) and technical loudness (dBFS) are two different things. Perceptual loudness is comparing two songs, and determining that one is louder than the other by the way they sound. Technical loudness is what the meter in the DAW is telling you (as in: you can’t go higher than 0.0 dBFS).
If you want to increase the perceived loudness of your audio, this is where compression/limiting comes in. By compressing the dynamic range of the signal (lowering its technical loudness), you can bring up its perceived loudness (RMS) more than before. (That’s the Make-Up Gain parameter in the plug-in.)
Be careful with the attack time – too short, and it will destroy the transient. Too long, and the peak might not be lowered at all. This may be the desire tonal quality, but will limit how much make-up gain you can apply.
Be careful with the release time – too short, and there will be audible “pumping”. Too long, and the compressor won’t be fully reset by the time the next transient triggers the compressor again.
You MUST adjust the threshold appropriately! While a preset might be a good starting point, the threshold parameter needs to be adjusted for your recording. Too high, and there may not be enough gain reduction. Too low, and it’s as if you just turned the volume down.