Suppose you were trying to limit access to a football match being held on an open park in a populous city. Without a fence, gate, or moat, you could not limit who could see the game. But suppose you had super powers and could cloak the players in invisibility uniforms. You would issue special glasses only to people allowed to see the match; others might look but see nothing. Although this scenario is pure fantasy, such an invisibility technology does exist, called encryption. Simply put, encryption is a tool by which we can transform data so only intended receivers (who have keys, the equivalent of anti-cloaking glasses) can deduce the concealed bits.
Charles, P. P., Shari L. P., Jonathan M. (2015). Security in Computing: Fifth Edition (pp. 37). Upper Saddle River, NJ: Prentice Hall.