I've always understood it as the perception that something isn't quite right (usually with a person, but I've seen it used in non-human contexts too) without being able to immediately describe why.

A great example is Grand Moff Tarkin in Rogue One - the actor who played him on the original trilogy died in 1994, so they just deep-faked him into the scenes he was needed. When I saw Rogue One initially, I didn't know that actor was dead, and didn't connect the dots that even if he wasn't dead, he'd look like a zombie this many years after filming the OT... but in Rogue One, he just looked like Tarkin. Mostly. The scenes that featured him gave a kind of uncomfortable "what the hell is wrong with that guy" feeling, but I still didn't connect the dots and couldn't put my finger on why it looked so wrong.

Then later I learned is was a deep-fake, and now it just looks like a deep-fake; the uncanny valley sensation went away once I finally understood why he looked the way he did.

The internet is full of creepy looking 'examples' of uncanny valley, but they're all shit imo, cuz they're all just blatantly creepy shit; well beyond the uncertainty that goes along with uncanny valley.