A drum tigger..aka Electric drum, is a pad that when struck, sends a message to a "brain" unit tha tells it to sound the drum sample that has been assigned to that pad.
Drum triggers can be hooked to samplers or sound units to get many kinds of sounds-you can assign a thunderclap to one if you want.
The big deal everyone is freaking out over them is that they are used often when drummers do hyperfast bass drum rolls. VERY few players can go hyper fast on bass drums and get clarity; therefore, to help get that clairty, triggers are used.