Like, say, you are a revolutionary who’s imprisoned, but you are allowed to write letters to your family. How would you covertly send a message to your fellow revolutionaries that only they can decode? (Your family are on your side will pass the letter along)
Obviously, it cannot appear to contain any obvious ciphertext, since the prison guards would just rip up your letter and not send it.
The concept for this is called Steganography and that article has several examples, but most would be difficult without tools/references.
The easiest would be stuff like using certain letters of words or word lengths as mentioned in some other comments here.