Sounds like a clown, but it actually goes inside a light!
A gobo is a metal or glass disc that fits inside a lighting fixture and allows for the projection of the image on that disc.
There are two main types of gobos. The first is metal. This is simply a piece of high-temperature metal that has had sections laser-cut out of it to make a shape. Think of a stencil that you might have made in art class at school, but much more accurate and much smaller. Because of the makeup of metal gobos, perfect circles are not possible, as they must have a join at some point.
The second type is a glass gobo. These allow for significantly more detail and allow for circles and other shapes. Glass gobos can have multiple layers that allow for multiple colours. Glass gobos are more expensive than metal ones.
Once designed and manufactured, the gobo is placed into a lighting fixture. Where there is a lack of metal or etching the light will pass through and project onto a surface. Gobos can be placed into a range of light fixtures from moving lights to generic lights. Gobos are typically used to project logos and or patterns across surfaces such as walls, furniture areas and other objects such as cars, stands, stages, etc.