"Mother sauce" refers to a basic sauce that's the starting point for making other sauces. For example, Béchamel is the base for many popular white sauces, cream sauces and cheese-based sauces like the ...
Delia' shows you how to make béchamel sauce, using a mixture of butter and flour called a roux. A béchamel sauce is one of the French "mother sauces" and is a really good one to learn.