This sets the signal level at which the compressor begins compressing your audio. Compression does not occur until the signal level reaches the threshold, and compression will cease once the incoming signal level drops below the threshold.
Your ratio control lets you decide how much compression is applied once the signal crosses that threshold — the actual ratio being referred to is the input level compared with the output level. At a ratio of 2:1, for example, every 2dB of signal level over the threshold will be compressed to deliver only 1dB of signal level increase at the output stage.
The attack control on a compressor determines how long it takes for the compressor circuit to reach (nearly) full compression — these are often very short values time-wise, sometimes down into the microsecond range. A good way to look at the attack control is that it determines how much of a sound’s initial attack will be compressed — longer attack times allow more of the initial attack to slip through before compression begins.
Similar to attack, the release control determines how long it takes compression to stop once the incoming signal level falls below the threshold. Setting this control too short can cause the compressor to “pump” or “breathe” — setting the release too long can cause equally unnatural sound effects.
Makeup Gain/Output Level
Because compression essentially lowers the volume of your audio in a dynamic, intelligent way, you may need to apply some makeup gain to bring the signal level back up after compression occurs. Use the output level or gain control on your compressor in conjunction with its bypass button to quickly compare and match the level of your compressed audio to the incoming signal.
Above ref from Sweetwater – Read full article on beginning compression – Sweetwater Article