A trigger is that part of a gun that when it is pressed, it causes it to be fired. However, a trigger is also something that sets off feelings of trauma. Triggers that can set off feelings of trauma could be anything. It could be images from pictures, videos or other forms of images.
These images could stir up a flashback which could take you back to memories of events. Sometimes those events could bring sad memories which might leave one depressed.
What acts as a trigger differs from one individual to another. For instance, when sad memories are triggered off by certain events, images, and so on, the individual involved may start avoiding those things that act as stimuli to those memories.
It is very important to identify those things that are your triggers, acknowledge them and deal with them.