What's the difference between Request RPM and Impression RPM on Admob?

Title says it all

Request RPM : your earnings for 1000 requests
Impression RPM: your earnings for 1000 impressions

I see now so the requests RPM is kind of like what you would have got if the fill rate was 100%

Request is when you request an ad.
Impression when you actually show it.

Impressions * Impression RPM = your earnings.

No, the request RPM is simply based on requests, so if you request an interstitial ad and then show it once, you’re R RPM and I RPM would be the same.