When you want to display an ad in your mobile app, you need to send a request for that ad. This request is called an “ad request”, and it is the first step in the process of displaying ads in your app using Google AdMob.

Google AdMob is a platform that provides an efficient ad mediation service for mobile app developers. Ad mediation is a process in which multiple ad networks bid for ad space in an app, which maximizes the revenue for the app developer. Whenever an app developer integrates AdMob into their app, ad requests are sent to AdMob, which then offers the ad space to multiple third-party ad networks in real-time through auctions.

An ad request is sent by the app developer's mobile app to Google AdMob servers, asking for an ad to be displayed. This request includes a range of parameters specific to the ad format and display context, including the ad size, placement, and targeting parameters. The Google AdMob server then processes these parameters and uses them to select an appropriate ad from its available inventory.

The ad request is an essential step in the ad serving process, as it enables an app developer to offer their available ad inventory to Google AdMob. There are different ad formats available on AdMob, including banner ads, interstitial ads, native ads, and rewarded video ads. 

To make an ad request, you need a few things:

1. AdMob Account
You need an AdMob account to submit ad requests. It is free to create an account and join the platform.

2. AdMob SDK Integration
 AdMob SDK is integrated into the mobile app to enable communication between the mobile app and AdMob servers. To get started, app developers need to integrate the AdMob SDK into the app. This involves adding some code snippets provided by AdMob into the app's source code.

3. Ad Unit ID
An Ad Unit ID is a unique identifier for a particular ad placement in an app. You need this to specify where you want to display the ad in the app. AdMob gives you a unique identifier for each ad unit you create.

4. Ad Targeting
By specifying attributes like location, demographics, and interests, an app developer can target their ad requests more closely to their audience. By narrowing the audience, you increase the chances that the ad inventory will be well-received. 

Once these components are in place, the app can start to make ad requests. When an ad request is made, an auction occurs in real-time through AdMob's mediation network, during which several ad networks compete to fill the ad unit. The winning ad from the highest bidding network is then returned to the app and displayed in the specified ad placement. 

It's highly recommended that the number of ad requests per app is monitored and controlled as this number directly affects your app's performance and fill rates. High fill rates with non-intrusive ad placements will lead to better user engagement and revenue. Moreover, adhering to AdMob's ad content policies can also ensure that your ad requests will generate an ad network fill rate that is favorable to your app.

In conclusion, ad requests are the first step in the process of displaying ads on mobile apps using Google AdMob. Ad requests are made by the app developer within the app, and these requests include information about the ad format, placement, and targeting parameters. Google AdMob then uses this information to offer the ad space to multiple third-party ad networks in real-time through auctions, with the network that wins the auction filling the ad unit and displaying the ad in the app's specified placement. By monitoring the number of ad requests and adhering to AdMob's policies, app developers can ensure high fill rates with non-intrusive ad placements, which will lead to better user engagement and revenue.