Hi, I'm Matthew.
You asked what the physical meaning of the slip/no-slip condition means.
You need to look up the term "Knudsen Number".
This is simply a reminder of what CFD is base upon.
This will give insight into your question.
Fluid Dynamics is mostly based on Continuim Mechanics which assumes that the properties of a gas or liquid can be approximated by some very simple rules.
These would be: