A mother color is a color you use in every mixed color in a particular painting. You can either use it as the starting point for the colors you create (mixing another color into some of your mother color) or mixing a little of your mother color into every color. The logic behind doing this is that it harmonizes all the colors in an individual painting, creating a unity in the color and composition.
A mother color can be used as a dominant color (or color theme) within a painting, or it can be used less prominently. A danger with using a mother color too strongly is that the colors are too similar (in tone and hue).
If you're working with glazes rather than physically mixing colors, you can use a mother color as a layer in the color you're building up. A final glaze with a mother color may be just what a painting needs to pull its components together.
You can select whatever color you wish as a mother color. It can be a neutral gray or brown mixed from the leftover colors on your palette, or it can be a straight-from-the-tube color.
