Allows you to edit and alter the palette present in an indexed image. Remember that you can only paint with the colors present in your indexed palette. This is why it's very handy to be able to add or alter colors in the palette.
Choose the color that you want to alter by clicking on it. Choose edit from the Operation menu. This will bring up the GIMP color selector which enables you to pick a replacement color.
If your indexed image contains less that 256 colors, then you can add a color. Choose Add from the Operation menu. A new color will appear as the last color in the palette. The new palette color is a copy the previous color in the palette. You are now free to alter the color and use it in your image.