CPM measures the cost per 1,000 impressions. It’s a good metric to evaluate competition level and costs to reach your audience. Oftentimes, CPM is the primary driver to a rise in costs.
Multiple factors contribute to your CPM (country, placement, competition for your audience, engagement on your ad, audience size, and more), but a high CPM is difficult to overcome and maintain a profitable CPA (Cost Per Action). That said, a low CPM isn’t always desirable as it may be reflective of reaching a lower quality audience.