The uncanny valley is the unnerving quality of things that are just slightly not right.
The concept was first hypothesized with respect to the field of robotics when professor Masohari Mori commented on robot aesthetics. The more humanoid a robot looks, the more we like it, until it reaches a threshold where the robot looks too human but not identical to a human. This point is the uncanny valley, where robots look creepy.
We can delineate certain stages:
Stage 1: The thing looks nothing like a human. It provokes no emotional reaction.
Stage 2: The thing looks like a cartoon, an abstract representation of a human. It provokes a positive emotional reaction.
Stage 3: The thing looks almost like a human, but not quite. It is creepy or unsettling.
Stage 4: The thing looks exactly like a human. We respond as if it were a human.
The uncanny valley is often used as a shorthand for something that is just slightly off or bad, where being technically worse would be better than just-less-than-perfect.