Uncanny valley is a concept.
Basically - Non human things with a few human features look fine.

For example - Wall.E. is obviously not a human but has eyes and arms like a human, and shows emotions.
But when you keep adding human features to that non-human thing, at one point you're unable to distinguish whether it is HUMAN or NON-HUMAN and at that point, it comes off as creepy.
Examples -


But if you keep pushing through and adding more features and keep making it more human-like, it gets out of the "valley" and it's again not creepy anymore.
The graph

You can look up Youtube videos for more explanation. I simplified as best as I could.