Application of a wood conditioner, thinned linseed oil or thinned shellac to wood prior to staining. You would apply a washcoat to wood that might blotch when stained with a liquid stain.

a coat of thinned finish that's applied to bare wood to partially seal the surface before a stain is applied

a thinned finish that will partially seal the wood but still allow the stain to penetrate

A reduced solids sealer that is applied over bare wood to reduce stain penetration or over stains to protect them from subsequent coatings.

Very thin coat of shellac or sanding sealer or coating.