This word is common in theatre, film, and television but appears here because it looks so odd and unEnglish. It is most often used by lighting technicians to refer to a metal plate with a pattern of holes in it, which is placed in the gate of a spotlight to produce an image or outline on the set. More sophisticated gobos rotate to create moving patterns; some are of glass with complex coloured patterns on them.
The word can also be used for a fabric or wood shape placed in front of a light to cast a shadow (in some circles in the US this is called a cookie or a flag instead). But it can also have other senses that relate to some sort of mask: perhaps a shield used to shelter a microphone from extraneous noise or to acoustically separate groups of instruments in an orchestra, or a screen used to shield a lens from light.
It has nothing to do with the Japanese vegetable of the same name — its origin is somewhat obscure, but it’s most probably just a condensed version of go between.
Another name for certain types is cucoloris. This is the usual name for a large perforated gobo placed in front of a lamp to project a diffused shadow pattern. It would seem to be the origin of the abbreviated form cookie that I mentioned above (if so, the latter would have no biscuit links). But where does cucoloris come from? My sources are totally silent on the matter — no dictionary I’ve consulted has even heard of the word, not even the big Oxford English Dictionary. Could it be from Latin cucullus for a hood or cowl?