Stains are fired blends of metal and ceramic oxides that have been reground into a fine powder. Stains containing otherwise toxic oxides can be employed without significant dangers. They are most popular at lower temperatures where colors tend to be brighter. However, most stains can also be used right up to the high fire.
Basically: the more stain you add, the darker the color will be.
For more information about stains, head on over to ourHow-to. In there, you can find so many more useful tips about stains, clays, and everything else about ceramics. Tip No.120is particularly important about how to use stains.