Apple posted about this iOS change in data privacy and tracking permissions here, or you can read the paraphrased paragraph below:
Starting with iOS 14.5, iPadOS 14.5, and tvOS 14.5, users will need to grant permission through the AppTrackingTransparency framework to track them or access their device’s advertising identifier. Tracking refers to the act of linking user or device data collected from the app with user or device data collected from other companies’ apps, websites, or offline properties for targeted advertising or advertising measurement purposes.
In the Airthings iOS app, there are some external links to other resources available to you:
From iOS app version 3.8.7, you can either grant or deny tracking permission when accessing these links. Accepting tracking permissions will lead to an enhanced user experience, in the sense of being able to provide you with more relevant content in future.