What is the unit of compressibility in Blender?
Almost anything can be compressed under the right conditions regardless of whether it is a solid, liquid or gas. Put a different way, if you apply pressure to a liquid, gas, or even many solids their volume will decrease. The amount that the volume will decrease in response to (a certain) pressure is called "Compressibility".
The Blender physics engine can simulate this phenomenon to a limited extent, and the particular value you are asking about is the amount of compression due to gravitational pull. The documentation for this feature is here.
I hope this helps you.