There are two types of ad sources that you can set up in your account: bidding ad sources or waterfall ad sources. The type of source that you set up determines how the ad source bids for your impressions. Bidding sources place bids in real time and compete in a single auction, while waterfall sources are called individually in the order of their eCPM, which is set by you.
You can also set up a custom event. Custom events let you set up waterfall mediation for an ad source that isn't supported by AdMob.
Learn how to: