In the past the noun “know” was sometimes used to mean “knowledge” or “knowing.” In the late 19th century the phrase “in the know” was sometimes used to mean having knowledge. Nowadays it is mostly used colloquially to mean, as the Oxford English Dictionary says, “in possession of information which is not generally known.”
It implies that there are others who are NOT in the know, who haven’t been clued in, who don’t know what’s going on.
You have now joined the “In The Know” crowd! Welcome to “In The Know With Sophia Lenore”.