Wax Resist (noun) is a waxy substance used to prevent slips, engobes, or glazes from adhering onto the clay body or a prior coating of slip, etc. A resist is especially helpful in keeping glaze from adhering to a pot's foot, and in the case of a lidded jar, from keeping the areas the lid and the jar meet free from glaze. In the past, wax resists where simply melted wax; today there are a number of wax emulsions, including many which are water-based.
Wax resist can also used in decoration. The resist can be used both under or over underglazes, glazes, and slips to create designs. These decorative elements can be quite simple or incredibly complex. Further information and examples can be found in the following articles: