Lingo is a way of speaking that's shared by a particular group of people — it's their own personal slang or jargon. You might observe International Talk Like a Pirate Day by trying to speak only pirate lingo.
Sometimes people refer to the language or dialect spoken in a place as its lingo: "I'd love to visit Paris, but I don't speak the lingo." You're more likely to hear lingo in the context of the words and phrases one group understands, but that outsiders might not, like computer lingo or English major lingo, or musician's lingo. Lingo shares a Latin root, lingua, or "tongue," with words like language and linguist.