Native app development is when an App is built for specific operating systems (OS), using the development tools and languages that the respective OS supports.
For Android coding nativly means, building the App in Android Studio using languages such as Java or Kotlin. While, for iOS it means building the App in XCode with Swift (or Objective C).
There was once a period when the scarcity of applications on the App Stores meant that almost any newly created app could find success, regardless of its quality. That era has passed. Today, the Google Play and Apple App Store brim with high-caliber applications, the result of hundreds or thousands of hours of meticulous development. To compete in this crowded and quality-driven marketplace, native app development is essential.
With Native Apps, the source code has to be written from scratch for each platform. While this creates better Apps, the development cost will be more and it will take more time to develop. In general it's sufficient to choose non-native development if your App will not be used by a lot of users or it's very simple.