It's where the creepiness of something spikes as it gets closer and closer to perfect realism, then drops again once it gets there. A cartoon is unrealistic but doesn't feel creepy because it's clearly
intended to be stylized. Bad CGI feels a bit off, but is still stylized enough not to be too uncomfortable. Good CGI that falls just short is uncomfortable because we know something subtle is off, but can't really place it. Then obviously real people are just fine because everything looks right.