Five-O, pronounced "five oh" rather than "five zero" and spelled with the letter O instead of the numeral 0, is a slang term used to indicate the presence of police officers or other law enforcement personnel. It can be used more generally to refer to specific police units, or as a generic term for all police — similar to cops, "the Man", "the Fuzz", or dozens of more pejorative slang terms. In the United States, the term Five-O is not considered to be derogatory, but it can be when used in other countries.
Believe it or not, this term originated from the popular American television series Hawaii Five-O (1968-1980). Hawaii is the fiftieth US state, and the program was a police procedural drama... and that's the long and short of it. That's how these two otherwise completely unrelated things, the number fifty expressed in this particular sort of "penal code" vernacular that law enforcement is known to use, and police themselves, became connected.