You app's display name is the name the user sees next to your icon on their screen. By default, NativeScript sets your app's display name based on the value you passed to tns create, which is oftentimes not exactly what you want the user to see. For example, running tns create my-app results in an app with a display name of myapp.
To change that value on iOS, first open your app's app/App_Resources/iOS/Info.plist file. The Info.plist file is iOS's main configuration file, and here you'll find a number of values you may want to tinker with before releasing your app. For display name, you'll want to alter the CFBundleDisplayName value.
Here's what this value looks like for nStudio: