In one line
CPM (Cost Per Mille, "mille" = thousand) is the cost per 1,000 impressions — the closest thing to the raw media price a platform charges.
Why it matters
When CPM rises, it usually means the auction got more competitive (seasonality, competitors upping bids) or your targeting got more expensive. Since CPC, CPI, and CPA are all downstream of CPM through CTR and CVR, tracking CPM on its own helps isolate one specific cause when costs rise.