A trigger is a reminder of an old unresolved event or piece of an event. Something in your environment causes you to recall a similar experience, event, or memory. Anything seen, felt, smelled, tasted, heard or sensed can be a trigger. They aren't inherently negative. Something happy can trigger you to remember something sad or unresolved. Your mind is an association machine. It collects and catalogues things that you perceive around you like a web or mind map.
Although people try to avoid them, they can be incredibly helpful for leading you toward things that need additional resolution. Once resolved, those experiences carry less or even no more emotional charge. This frees up your energy and allows you to collect the things you have learned and create new meaning. Energy work allows you to deal with triggers without falling into the emotional hole because it's not necessary to recount each detail. Your mind has already made the connection. Energy psychology allows you to release whatever is coming up.