If you’re using remarketing to reach people who’ve used your mobile app, they see your ads:
while using other mobile apps on the same mobile or tablet device
while browsing and using Google Maps
when they enter search terms for products like yours on a mobile device
each time they re-use your mobile app
Remarketing lets you show ads to people who have installed your mobile app, and there are a couple different types of “Mobile app users” remarketing lists. In general, they allow you to show ads to people who are already users of your app while they use other apps on the Display Network.
Remarketing lets you show ads to people who have visited your website or used your mobile app before. When people leave your website without buying anything, for example, remarketing helps you reconnect with them by showing relevant ads as they browse the web, as they use mobile apps, or as they search on Google.
Remarketing for mobile apps: Show ads to people who have used your mobile app or mobile website as they use other mobile apps or browse other mobile websites
Read more here: https://support.google.com/adwords/answer/2453998?hl=en