Fluid Interfaces are design principles announced at WWDC in 2018. As you would expect, these principles guide you on how to prioritize your app for the user experience. In this post, Mats has some great examples of what is and isn't considered a good fluid interface.
Fluid Interfaces is all about prioritizing the user. We want the user to understand what the system is trying to communicate, and we also want the user to feel control.
Fluid Interfaces by Mats Hauge