I appreciate your eagerness to learn how things work! Here are the definitions you are looking for.
Impressions- This is how much times your ad (adsense ad) loaded by the visitors. For example, when a visitor opens a page of ISC (supposed to be the page you have contributed), the ads on that page will also begin to load. If there are 4 ads, then 4 is the impression. If 100 pages opened with 4 ads, then the impressions will be 400.
RPM- This is revenue per mile. It is calculated as how much your ad can earn for 1000 page views. RPM is a prediction only based on CPC which is cost per click. CPC is the real earning. CPC will also vary for different ads. It is pre-set value of the advertiser who owns the ads.
Impression RPM- This is similar to RPM (Page RPM). It is a predicted value of revenue for 1000 impressions. Some advertisers may pay money for impressions only, not clicks. For example, they may set like $0.2 per 1000 impressions.
Hope you have understood! If you have any more doubt, don't hesitate to ask.
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