Somewhere I read (Schopenhauer, I think) that boredom occurs when you feel time pass while doing fruitless activity. Unlike pain, which invades our sensations, forcing our response, boredom seeps in from contrasting our current action from a better alternative.
Existential boredom, then, is a sense that life more generally is not fulfilling. It might be “pleasurable” at a daily level, but when one steps away, it lacks something. Life–like a story–needs a certain cohesion, a “meaning” or significance.