The noun parlance means the manner of speaking and using words. Your 15-year-old will tell you that you are definitely not fluent in the parlance of teenagers, no matter how cool you think you are.
Parlance comes from the French word parler, meaning "to speak." The noun is often used to refer to the jargon or slang a certain group or industry uses. If you work in the sciences, you'll be familiar with the parlance of scientists and be familiar with such jargon as "goodness of fit" and "iff", which means "if and only if". But if you are not a scientist, you may stumble when you first come across such terms.