A trigger lock is a safety feature that prevents the trigger from being depressed without the trigger lock being pressed down.   The idea behind the trigger lock is to help avoid something other than the chainsaw operators finger, pressing down on the trigger, like a branch or foliage.  In other words, the lock is in place to prevent unintentional throttle acceleration.   

It is important to research the functionality of the specific chainsaw trigger lock feature that you maybe considering to buy.  Many chainsaw users felt confused or unclear on how their specific lock operates and protects.