"Y/N" usually proceeds an interrogative or question made by one party asked to another party or individual. Simply, it means 'yes' or 'no'. Sometimes, individuals use 'y/n' for arbitrary means, such as adding 'y/n' to statements that are not interrogatives or questions.
Correct use of Y/N?: Should she get gastro bypass surgery, y/n?
Answer: Y as in yes if the person needs it and it's good for their health.
N if the person is borderline skinny or can possibly lose the weight without going under the knife, or just in general.
Incorrect use of Y/N?: She is so overweight she can barely move. The doctor said she needs gastro bypass surgery, y/n?