Gels are color filters--thin pieces of plastic--that you put over the light to change the color. You use them to add color to a scene, or to color-balance your flash with the ambient lighting so that white balancing doesn't become a mixed-light juggling act.
For example, if you're shooting under tungsten lighting, you add orange to your light so that when the tungsten white balance is applied to move everything more towards blue, the bits of your image that have flash illumination doesn't become overly blue (as they would if you hadn't made it all orange-y). If you're shooting under flourescent, you add green to your flash so it matches the color of the floruescent.
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