Pattern colors are a special kind of color that represents a repeating image pattern. They are useful if you want to have a repeating pattern drawn in part of your image. The pattern image is scaled so it looks the same at all resolutions.
You can create pattern colors from a part of your image by selecting the Format > Select Pattern Color menu item and then selecting a rectangle in your image. This part of the image will become a pattern color which is assigned to your image's primary color. If you want to save a pattern color to use later, you can drag it into the color wells in the Color Inspector just like any other color.
Note: There are a few places you can't use pattern colors: in gradients and filter attributes.