CPM stands for Cost-Per-Mille or Cost-Per-Thousand. It is a digital marketing term for the calculation that measures the price of 1000 ad-impressions (or ad-displays) on a single webpage.
Bidding systems use CPM to calculate the cost of an advertisement for every 1000 people viewing it.

e.g. If an advertiser has a budget of $20 per day and wants to have 4000 impressions of his ad on that particular day, he must place the bid at a CPM price of $5.