eCPM expresses ad revenue as a rate per thousand impressions, which lets you compare formats, networks, placements and countries on a common basis.
It is useful but incomplete. A demand source can show a high eCPM while contributing little, because it rarely bids, rarely wins, or renders poorly. And raising price floors raises observed eCPM mechanically while cutting fill. For decisions, pair eCPM with fill, show rate and revenue per eligible opportunity.
eCPM also varies widely by geography, platform, format and season, so a blended eCPM mostly reflects your traffic mix.