eCPM, or effective cost per mille, is a metric used in mobile advertising that represents the revenue earned for every thousand impressions of ads served. It is commonly used by mobile app developers, advertisers, and ad networks to measure the financial performance of their ad campaigns. 

In Google AdMob, eCPM is calculated by dividing the total earnings generated in a specific period by the number of impressions in that same period, multiplied by 1000. The formula can be represented as follows: 

eCPM = (Total Earnings / Number of Impressions) x 1000

For example, if an ad unit generates $1,000 in revenue from 100,000 impressions, the eCPM would be $10. 

eCPM is a useful metric for app developers as it allows them to compare and evaluate the revenue potential of different ad networks, ad formats, and ad placements. eCPM also helps in identifying the areas that need improvement in the ad campaign. For instance, a low eCPM may indicate that the ad is not targeting the right audience or that the ad placement is not optimal. 

Here are some factors that can impact your eCPM in AdMob:

1. Ad Format
Different ad formats can have varying eCPMs. For example, interstitial ads generally have higher eCPMs than banner ads because they are more intrusive and have higher engagement rates. Knowing which ad format works best for your app can help you optimize your strategy for higher eCPMs.

2. User Demographics
User demographics such as age, gender, interests, and location can also play a significant role in eCPM. Advertisers are willing to pay more for ads targeted at specific user groups. By using AdMob's targeting tools to ensure that your ads are shown to the right audience, you can increase your eCPM.

3. Ad Placement
Ad placement can impact the eCPM of your ads. Placing ads in highly visible positions in your app with more user engagement can help to increase the eCPM. Experimenting with different ad placements can help you identify the placements that perform best for your app.

4. Ad Frequency
Overloading the user with ads can lead to banner blindness and decrease engagement with the ad. Consequently, lower engagement can result in reduced eCPM. App developers should aim to find a balance between generating revenue from ads and making sure that they do not annoy their users using too many ads.

5. Quality of Ads
High-quality ads are likely to attract more clicks and user engagements. The quality of ads and content can, therefore, significantly impact the eCPM. By using high-quality ads, app developers can ensure performance improvements such as higher click-through rates, engagement, and eCPM.

In conclusion, eCPM is a key performance metric that is used to evaluate the success of mobile advertising campaigns. It enables app developers to measure the revenue generated on a per-impression basis and make informed decisions concerning their ad campaigns. AdMob provides tools to track impressions and eCPM, and by optimizing factors such as ad format, user demographics, ad placement, ad frequency, and quality of ads, app developers can improve their eCPM and maximize the revenue generated from their app.