It's the tendency for people to overestimate the intensity or the duration of future emotional states.
Research shows that people predict the valence, if it's good or bad, but are less accurate in their predictions about the duration and durability of these effects.
This bias is correlated to the pursuit of happiness. People can think they will be happy they have something. But they overestimate the intensity of the effects that such a thing brings. So when they acquire that thing, they won't be as happy as they thought.