https://docs.microsoft.com/en-us/dotnet/api/xamarin.essentials.displayinfo.density simply says:

Gets a value representing the screen density.

and

Property Value
Double
The screen density.

No word about the unit or how to interpret this value at all.

Looking at the source code (https://github.com/xamarin/Essentials/blob/main/Xamarin.Essentials/DeviceDisplay/DeviceDisplay.uwp.cs#L60) one can guess that the number represents multiples of 96 dpi, at least on UWP. No idea if it will be the same on all platforms (but I certainly hope so).