Here you can find short explanation about trigger skills in the Torchlight: Infinite game.
Trigger Skills Explanation
It’s passive skills, stuff like cast on crit, cast while channeling, curse on hit. All of those are triggers, and then whatever skill/spell you apply in its modifier slot is the trigger skill.
You can have conditions that when satisfied automatically trigger your skill. For example you setup a trigger support skill on your ice lance that says cast when damaged and then whenever you are damaged then the ice lance will cast.
It lets you automate part of your build so you can multi-task and be even stronger.