The cost to purchase 1,000 impressions on an ad platform. "Mille" is Latin for thousand. It represents the market price of attention.
CPM = (Total Ad Spend / Total Impressions) × 1,000
CPM is driven purely by supply (available ad space) and demand (number of advertisers). You cannot directly "fix" high CPMs, but you can understand them.
CPMs historically rise in Q4 (Black Friday/Christmas) as big brands flood the market with budget, and drop in Q1 (January) when spending cools off.
Platforms like Meta and TikTok reward engaging content. If your ad has high engagement (likes, shares, watch time), the platform effectively subsidizes your CPM, giving you cheaper impressions because your ad improves the user experience.