I believe I can help with that:
Requests are the number of times your app contacts the advertising service and asks for an ad.
Impressions are the number of times your users have actually seen the ads.
Fill Rate is the percentage of your requests that are actually filled. Advertising services do not have an infinite supply of ads, so sometimes (or in the case of BBAS most of the time) your app requests an ad and there isn't one available.
Clicks are the number of times your users have clicked on an ad.
CTR is Click-through-rate, which is basically Impressions divided by Clicks. It shows how frequently your users interact with your ads.
eCPM is effective cost per thousand impressions. It's generally an estimate of how much money you can expect to make for every 1000 views your ads get.
