What is CPM?
CPM (cost per mille) is the price for 1,000 ad impressions.
Meta Ads and display campaigns are usually bought by impressions. CPM tells you how expensive it is to reach an audience, not how many people act.
To compare with search, convert CPM to cost per click using CTR. A cheap CPM with a very low CTR can still mean expensive clicks.
Formula
CPM = ad spend / impressions x 1,000
Example
You pay 50 EUR for 10,000 impressions. CPM is 5 EUR.