I'm glad you're finding this place friendly
It's what we're about.
'Triggering' is a term used to describe something, in this case a thread or topic, that would cause other users to have a strong emotional reaction that may make them feel the urge to hurt themselves. So a 'triggering' thread is one that has content that may cause people to have violent emotions. It's used as a warning, so users know that if they are sensitive to the material, that they have an opportunity not to read it.
Hope that helps