A CMP is a Cost per Mille, or Cost per Thousand, signifying the cost an advertiser is willing to pay for one thousand advertisement impressions.
In other words, a CMP is a way to price online advertising. Advertisers can choose to bids on a CPM basis, meaning they'll pay a certain price for every thousand ad impressions. Often, ad networks will also allow advertisers to set a maximum CPM, meaning they'll only pay that price for ad impressions up to a certain amount.
CPM is often used as a benchmark to compare advertising prices, as it's a standard way to measure ad costs. However, There are a number of other pricing models out there, so it's important to weigh the pros and cons of each before settling on a price.