More Info on the term eCPM?

Hello,

I have searched through the Help and this Forum; however I am still not clear about the term eCPM (eCPM: Effective cost per thousand impressions).

I mean, cost to whom? Then, how eCPM is related to my estimated revenue - a bigger amount is better or a lesser amount is better? Is there a math formula like this one - {(Impressions/Requests) = Fill Rate}

Can somebody, with examples, may be explain the term - eCPM?

Thanks in advance!