CPM stands for cost per thousand impressions, it refers to the cost of your ad appearing 1000 times.

To calculate CPM, divide the cost by impressions and multiply by 1000

CPM = (cost ÷ impressions) × 1000

For example a $500 campaign at a $90CPM will potentially generate 5,555 impressions

$90 = ($500 ÷ 5,555) × 1000