Soldier, do you see that rifle in your hands? The trigger is that thing you pull to make the bullets come out the front end!
(Sorry, I couldn't resist)
Seriously though. A trigger is the amount of deviation from expectations that is needed to (hopefully) cause a tradeable market move.
As a completely bogus example, consider this:
At 4:30 pm today, the monthly Canadian Beer Production report is due out. Expectations are for 100 million bottles. The trigger for this is 10. If production hits or exceeds the high end trigger (110 million bottles or more), that would be good for the Canadian Dollar. If production hits or exceeds the low end trigger (90 million or less), that would be bad for the Canadian Dollar.