trigger, for me, means a series of words (or events), that disturbs your emotional state at the time.
I know for me, 1 of my biggist triggers is family, and when ever they are mentioned or talked about, it unsettles me and makes me feel unstable (not because of chemicals or anything like that, because family was mentioned)

perhaps an easier way to look at it is to think of people with PTSD

what triggers (starts off), their bad emotional reactions

events/ traumas from their past.

that's why they have it in the first place, because they've witnessed something that was traumatic for them
so if you try to look at it from that point of view, and then apply it to us, it works exactly the same.

family is a trigger for me because my mother abused me and cut me out of her life, so naturally when ever family is mentioned, it starts off a neggative reaction

triggers can be anything though. sounds, smells, looking at things, certain people in the room, another one for me is a baby crying.

you can also get good triggers. like, 1 of my good triggers is watching the disney movie the lion king. it makes me feel good, and makes me feel safe