Gels are thin translucent sheets of “plastic” used to alter the color of lights and strobes. They can be used for interesting effects like adding a colored background to a portrait, or to “correct” the lighting when multiple kinds of lights are used. For example, fluorescent lights, incandescent lights and speedlites all operate at different color temperatures. If you make a picture that includes all three, you could get an ugly mix of color-casts on the subject. For example, if you’re making a portrait in an office with lots of fluorescent lighting (and aren’t overpowering the ambient completely), you should add a “plus green” gel to your strobe so that it matches the color of the ambient fluorescent lighting. They also make gels that go around fluorescent tubes (“minus green”) but it’s much easier to just gel your flash.