The labels have three categories: Data Used to Track You, Data Linked to You, and Data Not Linked to You, with bullet points for each detailing what the app has going on under the hood. A label might reveal that an app wants to collect your location data, financial details, and contact information, and links all of that to an in-service account or identifiers like your device's ID number. The label might also show that the app goes a step farther and shares that information with other companies to track you across their websites and services as well.
Many apps that have submitted information will have their labels go live today, but it will take some time before they become universal. The privacy details are only mandatory once a developer submits a new app or an update to Apple for review, and many apps have infrequent update cycles. Apple says, though, that some developers have added the information anyway, perhaps to avoid the appearance of withholding something.
In the reality of today's app landscape, it's difficult to find mainstream software that doesn't do at least some linking and tracking. The privacy labels will help drive that point home, but that pervasiveness might also make it hard to find something actionable in the information. And while providing data for the labels is now mandatory in the iOS and macOS App Stores, it's also the developer's job to provide factual information and revise it over time.