Flags are simply a black piece of material or foamcore that is used to control the spill of the light. This may be to keep light off of the background or a particular subject. It may also be to keep light from shining directly at the camera creating lens flares. Whatever the purpose, it is not unusual to see several flags of different shapes and sizes being used to control a single light's output. It is helpful to know that the edge of the shadow created by the flag will be softer the closer it is placed to the light. Gels are generally used to color-correct a light to match the other lights in a scene. They can also be used for adding color as an effect in a scene. Scrims are diffusion material that comes in different thicknesses. They are used for lowering the light level and for softening the source by making it bigger. Finally, cookies are cutout shapes that are placed in front of a light to "break it up" and cast a more random pattern of light. Whether used on the background or on other elements in a scene, this tends to make a scene more interesting and often helps it to feel more natural. In the above example from a production by the company Encendedor, you see quite a few light-control techniques being used, including a scrim placed directly on the front windshield of the car.
