CPM is the cost of being seen 1,000 times, whether or not anyone clicks. It's the rawest measure of media pricing and lets you compare how expensive it is to reach an audience on Meta vs. TikTok vs. Google.
Example: $5,000 spend ÷ 500,000 impressions × 1,000 = a $10 CPM. Cheap reach to the wrong audience is still wasted money, so always read CPM next to CTR and conversion rate.