Ritual garb is the clothing that you wear for magical or ceremonial purposes. It is a shorthand, general term and its precise meaning varies greatly by tradition or individual.
Ritual garb as compared to street clothes is any outfit or even a single article of clothing or jewelry that sets one apart while wearing it, separating the sacred from the mundane by the wearing of the item. Some traditions prescribe full Ceremonial garb, complete with rituals surrounding their donning and removal, storage and cleaning and who can wear what under what circumstances. Most are not that complicated. Some prescribe certain colors, leaving the individual free to choose the garment, some prescribe specific items of clothing, such as cowls, robes or cloaks and others suggest pieces of jewelry.
Generally, when an advertisement for a public event suggests wearing ritual garb or ceremonial garb, it is up to individuals to define it according to their own traditions unless it says something like “Ritual garb appropriate to blah blah tradition”, then you may need to do some research. If the advertisement says “street clothes” or “come as you are”, then you can wear whatever you like, including ritual garb. Many like to have a somewhat toned down ritual garb or perhaps just a special piece of jewelry for these occasions and reserve the full regalia for more intimate occasions while others feel that if you’re doing ritual you should be in garb, every time. Your own aesthetics and the rules of your tradition will define this for you.