my understanding is that the word trigger is used in psychology to describe a wide range of traumatic responses. e.g whenever a war veteran suffering from ptsd hears firework it may trigger a strong traumatic response. //victims of abuse can get "triggered" by the presence or even mention of their abuser etc
it applies to a wide range of traumas and responses some of which might be more or less extreme, so it does include some small things: if you ever get a minor burn, the idea of touching a potentially hot surface might make you somewhat uncomfortable - you could call that a triggering experience even though it's not nearly as intense as the other examples.
Triggering experiences are generally considered to reinforce trauma and generate unncessary distress and should therefore be avoided
And so we use trigger warning before movies etc to warn users of potentially upsetting/triggering content such as war, torture, sexual violence and various forms of abuse, it's really not that big of a deal