This is a great question. I'll start out by saying compression is a huge topic and there isn't necessary a best setting, though there can definitely be wrong settings.
Sometimes, maybe often times, the best setting is Bypassed.
"Threshold" is a setting that is entirely dependent on the source material. A more negative number will result in more compression. The closer the Threshold number is to 0 (which is the top of the scale in digital terms), the less compression there will be.
The top meter on the screen shows how much gain reduction is occurring. Roughly, each hash mark is about 2 dB of reduction. (No metering would indicate no compression, which means the signal is below the Threshold setting.)
Here's where I would start:
- Use the Light Compression setting
- Leave the Gain at 3.0 dB
- Set the Threshold so that on the louder parts of the performance, the meter at the top of the screen is hitting between the 1st and 3rd hash marks (starting at the right side of the screen). This would then mean that for the softer and mid-volume parts, there may be no compression happening at all. That's perfectly fine and probably desired.
Hope this helps,