A notation placed into a hyperlink online or placed at the beginning of a narrative, it serves the same kind of function as a post-commercial or post-bumper pause in a TV or radio show to say "The following content may be disturbing for sensitive viewers. Viewer discretion is advised."
Originally, it came out of the idea that a similar experience to a traumatic experience can stir up memories of a past trauma - whether or not the person knows about that trauma or not. The psychologist John Watson showed with operant conditioning that a child exposed (in the case of his test subject, "Baby Albert" - it was a white fuzzy animal) to a similar stimulus to one paired with an unpleasant experience reacts with fear and/or terror.
The book A Clockwork Orange had as a main plot device the idea of deliberately instilling a trauma to produce mind control.
It entered feminist and social justice narrative as a shorthand to explain that certain subjects should be given a sort of "spoiler tag" with respect to a potential audience. Or conversely, someone can pre-emptively share something that triggers that person - and out of courtesy and simple human decency, the topic will be avoided. Of course, for most people this was used in a reasonable and supportive way, but a minority of people used it as a debate tactic to shut down the other side of an argument. "These are words that cannot be said, this is your argument that cannot be made. Therefore I win."
I am extraordinarily thankful that I never mocked anyone for it.
Obvious trigger to obvious post traumatic stress disorder is obvious. I know Vietnam veterans who simply leave populated areas during Independence Day because to this day they can't abide sudden loud explosions. A Marine I know who served in the Gulf won't stay in a room with a woman wearing a hijab - which is usually taken as a sign of racism. Everyone's had that one girlfriend who has that one thing you can't do, because of some very bad memories.
But I found myself the other day having almost a panic attack in response to a ringtone. I drove a very familiar route this past weekend and found my vision blurring with tears, and to my horror I realized I hadn't even noticed I'd just passed the exit I drove to time and time again to try to drop something off to make my spouse's stay in psychiatric care more bearable. I have to take a few moments when I change a razor blade in the razor I use in my barbering apprenticeship.
And it was this that made me realize that there was far, far more to my feelings than I'd even realized. Something was terribly, terribly wrong.
It's so shocking how unexpected, surprising, and frankly gut-punchingly mood-changing such a thing can be. The bad news for me is that the ringtone in question is the default ringtone for a popular model of Android phone, so I hear it over and over again.
I deal with feelings of embarassment. There's no reason a sound should provoke such a strong over-reaction. There's no sane purpose behind being so affected by a memory of a time gone by - an event over. I'm ashamed at the fact that I'm so deeply affected by something that reminds me of something that didn't even so much happen to me.
But you can't reason with a phobia any more than you can reason with this phenomenon. Whoever named it "trigger" really knew their onions.