This is about works that, for the most part, never cross the line to bad or good. They just stay roughly in the middle, in the opinion of the viewer. These are the works we often refer to as "decent", "okay", "meh", and "mediocre".
Now this doesn't seem important, but some works can still do well even when the audience thinks this about a work. It's a form of comfort food, and often easier to pull off than trying to get the audience to think a work is awesome (which has a higher risk of being horrible if done wrong).
Why do viewers feel this way? Well it depends. Usually it's a Strictly Formula work that is competent, but little else. Often it's a work that is technically and artistically competent, but otherwise uninspired (this can also apply to works that have high production values, but just don't add up to a satisfying whole package). Once in a while, it's a work that strives for greatness, but the result or idea is too weak to get there.
Reactions can vary, depending on expectations. Those expecting something great can often feel just as cheated as if the work was outright bad. This is largely the reason for the Four Point Scale. On the other hand, if you aren't expecting much, it's a nice way to kill time, and that's all you are looking for. On yet another hand there's what Joe Queenan described as Scheissbedauer ("shit regret") which is when you were hoping for something that would monumentally SUCK, but it just turns out...meh.
Compare Capcom Sequel Stagnation (which often leads to this).
No examples, please. This is mostly a personal reaction anyway and shouldn't be put in main.