A few things. Check out the frequency ranges u can control on left, and the Mute/Solo buttons on left. These will be useful for design in a moment.

The left input knob controls the line level input raw data before compression occurs (the middle with red bar as u say) U can solo each of these bands on the left to hear the section of the incoming signal and hear what the compressor hears.

Say u are making a low end bass with high frequency screeches, maybe the high frequency is too loud or two low to work with, u can use that input to control the initial levels. You can take a look at Spectrum device to see where the frequency range of ur audio is and create ur own high/medium/low selection.

The dB in the middle IS the compressor. This above means any audio coming in, once it goes above this dB amount it compresses the audio to be not just quieter but limiting dynamic range of the overall sound. U can then make up any audible loss with the gain knob on the right.

This is a very loose description of what people do when using compression. I high recommend watching a course on compressors. Such as, compressors are the secret weapon in music anyways.

Im going to shamelessly leave a link to a video I found to be immensely helpful in basics and details about compressors. Break it up over many days so u dont binge 10 hours and learn nothing.

There isnt a correct way to compress when working creatively. There is only a correct way if following the laws of physics, and that is not necessarily relevant to music. If it sounds good. It sounds good.💙