Trigger release, in this case, refer to hand held release aids that use a trigger on the release. In most cases this would be activated using the thumb. There are some models that fire with the pinky finger or even index. However, as you will see, most use the thumb.
This style of release is how most people get into target archery, however they are also gaining more popularity in hunting as well.
The initial way most people use this style is to push the trigger with their thumb.