Private Relay is an attempt by Apple to change the way traffic is routed from user to internet service and back. This is designed to break the relationship between user IP address and information about that user, reducing the digital footprint of that user and eliminating certain venues of advertising information.
It is a new feature in the latest version of iOS and MacOS that will be launching in "beta mode". It is available to all users who pay Apple for iCloud storage and I became interested in it after watching the WWDC session about preparing for it.
Source: How does Apple Private Relay Work?, an article by Mathew Duggan.