Can someone please explain what is "Friction Slide multiplier" in CII Configuration setup ?

Per the User Guide:
"Specifies the internal friction sliding force multiplier.
IMPORTANT: You should never adjust value unless you are instructed to do by CAS support"

yes.. if we change it what will happen?

This is a tuning factor to adjust CAESAR II results to observed field behavior. The problem with this is that there are multiple equilibrium solutions once you include friction in a model. Convergence (and therefore results) vary based on the order of restraint convergence, which can vary by load case.

While this option makes numerical sense, it is difficult to use, hence our recommendation to NOT adjust this. (Actually we should remove this from the interface.)