To elaborate on Zeke's reply, when mic'ing various instuments you maybe don't want the mic to pick up other sounds - as an example, you don't want the tom mic's to pick up the sound of a guitar amp. So, you set a gate on the mic to only pick up sounds at a certain level. Again as an example, the tom mics will only "open up" when you hit the tom, creating a loud enough sound - any sounds below the level you set (such as the nearbye guitar amp) will not be picked up.
You may also hear about "gated reverb" which is a creative use of a gate on a reverb sound to allow a fast "big" sound but eliminate the long "tail" of the reverb.
I have never used Sonar, so I can't tell you if Sonar has gating capability or not.
