A mobile application programmed in the recommended language and framework of either iOS or Android. For example, Objective C and Swift are iOS languages, and Java is widely used to program Android apps. There are other languages as well.
The major problem with native mobile apps is that any change to the program's behavior requires changes to the source code of both platforms. Programmers may or may not be experienced in both iOS and Android because the two environments are dramatically different from each other.
Hybrid Mobile Apps and Web Apps
There are alternatives to native apps. For example, "hybrid mobile apps" are based on Web programming and take advantage of the abundance of Web programmers. There is less programming with hybrid mobile apps. See hybrid mobile app
Web apps run in the browser and are downloaded from a website royalty free. If a native app is not free, developers pay a royalty to the app store. See Web application
and native application