Triggering material would be anything that discusses death, abuse, bad language, self injury, anything that may upset someone. By using a trigger icon on a thread that "may" be triggering is always the safest way to go.

Some members get easily upset if they start reading a thread or post that has triggering information in it and they've not been warned by a trigger icon.

Here are a couple of links for you regarding the use of the trigger icon:

http://forums.psychcentral.com/rules...gger-icon.html

What is a Trigger? | Psych Central

I hope that was helpful for you!