What does it do differently than centimeters, meters, inches, micrometers, etc.?
Adaptive units will be displayed as different units depending on the size of the values automatically. For example if you enter 1000m to any length input field, the value will be shown as 1 km, however smaller values will be shown as smaller units like meters or centimeters and so on:
This is exactly the same behavior as in previous versions.
The new functionality in Blender 2.80 is that units can be set to one specific fixed unit and then all values will be shown as that specific unit so for example even if you enter 1km to some length input field it will show as 100000cm if you have the units set to cm: