Sometimes an ad network won't fill a request for an ad. This could be done for various reasons: there are no ads available for the requesting region, a user has hit their frequency cap with that network for the day, and many others. For this reason, it is best to work with several ad networks and implement "fail over" logic to try and fill a request that has gone unfilled by a network. For example, you might choose to show an AppLovin ad then fail over to another network if we don't provide an ad.
This setup is called a "waterfall" and it can help you increase your overall revenue since you won't have to rely on just one network for 100% of your requests. A simple implementation of a waterfall would be as follows:
if AppLovin has an ad available to show show the ad else ask another network if they have an ad available to show