Carding is a term for rubbing the surface with a course bit of cloth or, sometimes steel wool.
Cloth like burlap or denim is often used because its course weave tends to scrape the surface better than a tightly woven fabric.
I mentioned steel wool but it is usually reserved for carding the rust off of steel parts during the browning/bluing process.
I do not recommend using steel wool to card stains on wood.
The problem with steel wool on wood is small particles of the steel fibers will break off and become trapped in the woods grain.
If this happens and your stain has any water at all in it, when you apply the next coat of stain, the water will cause the steel fibers to rust, leaving tens of thousands of little freckles all over the surface.
Nobody likes to see a gunstock with a freckle complexion problem. :rotf: