It seems pretty clear in the "Detailed FAQ"

> We are introducing a Unity Runtime Fee that applies to certain Unity subscription plans based on per-game installs across any Unity-supported game platform.

> An install is defined as the installation and initialization of a project on an end user’s device.

> Each time a game is downloaded, Unity’s runtime code is also installed. The Unity Runtime Fee goes towards the continued investment in that code to support the billions of devices served every month.