Weighted Prediction: What is it?

I see an option for the H.264 and H.265 Master settings that is called "Use weighted prediction". I have looked up in the manual and online to find out what it is but to no avail. What does this checkbox do and should I use it in exports? It defaults to unchecked.

Google has many links to weighted prediction in H264 & H265 encoding.

I've searched it up on Google and it still seems sort of confusing. It apparently improves fades to black and white with less bitrate usage than without the prediction, so I guess I should have it on at all times?