BYOB is an acronym most commonly meant to stand for "bring your own beer", "bring your own bottle", "bring your own beverage" or "bring your own booze". The term "booze" is slang for alcoholic drinks. It is also commonly used to mean "bring your own beef" at a barbecue.
BYOB is often placed on an invitation to indicate that the host will not be providing alcohol and that guests are welcome to bring their own. It is also frequently used by regular bars, restaurants, or strip clubs which do not have licenses to serve liquor or alcoholic beverages in general; an alternate term for this is brownbagging.
Several other meanings have been used, but as they are neither widely used nor recognised, they are only understood when expanded. For example, "bring your own bagels", "bring your own bhang", "bring your own Bat'leth" (said in an episode of Star Trek: Voyager) or "build your own box" in hacker jargon. Overall it is a very popular acronym that is often used to mean different things by intent.