In the live environment, compression boosts the vocal and lead instruments in relation to noisier instruments like drums. The compression ratio determines the output level of a signal before and after compression. For example, a ratio of 5:1 means any signal five decibels over the threshold will be reduced to one decibel over threshold.
Attack is the length of time it takes the compressor to act. So an attack of 200 miliseconds would mean the signal hitting the compressor would peak for 200 miliseconds before being compressed. Too long of an attack creates unwanted peaks, too short of an attack can clip the beginning of a sound.
The release is how long the compressor increases the signal to the threshold before it is allowed to decay naturally. A quick release of no less than 100 miliseconds is most suitable for live sound. Anything slower would diminish the overall dynamics of the performance.