What is a slip coat when applying a wood finish?

A slip coat provides a very temporary barrier to apply stain over raw wood with less drag.

For example, we like to use a slip coat of mineral spirits when applying General Finishes oil-based Gel Stain to a raw wood surface to allow the stain to slice across the surface more easily. Or a slip coat of General Finishes Water-based Extender when applying our water-based wood stain.