Taking your questions in reverse order:
NS(Insert permission here)Description is where you provide the text that is displayed in the popup users are shown when you ask for a particular permission. For example, if your app needs access to the photo library, the first time the app tries to interact with the photo library, the user is shown a popup by iOS stating that the app is trying to access the photo library and do they want to allow that. Your NSPhotoLibraryUsageDescription text will be part of that. “This lets you save your paper doll avatars to your camera roll.” (Or whatever.) NSUserTrackingUsageDescription works the same way. Here’s Apple’s page about it.
The SKAdNetworkIdentifier is how advertisers track installs under Apple’s new privacy system. They provide you with a list of the SKAdNetworkIdentifiers they use for attribution and if you don’t include it in your build.settings, they have now way of tracking attribution and are unlikely to serve you ads that track installs as a metric of success.
Your players will only get one ATT popup and then they will never be shown it again. (Assuming they don’t uninstall and then reinstall the app.) If they want to change the tracking permission they’ve given your app, the have to go into iOS’s settings. Essentially, whatever they choose when they first see the popup is what they’re going to have going forward.
You can check out the ATT popup in your app by uninstalling it from your phone and then reinstalling it.