AdMob helps you monetize your mobile app through in-app advertising. Ads can be displayed in a number of formats and are seamlessly added to platform native UI components.
Before you can display ads within your app, you'll need to create an AdMob account and activate one or more ad unit IDs. This is a unique identifier for the places in your app where ads are displayed.
AdMob uses the Google Mobile Ads SDK which helps app developers gain insights about their users and maximize ad revenue. To do so, the default integration of the Mobile Ads SDK collects information such as device information.
Source: AdMob