In the world of cryptography, cipher is an algorithm or procedure or steps neeeded to encrypt or decrypt a text, sentence or word. Generally speaking, it is an approach to hide or unhide a single letter or char (character) for the purpose of information security, data security in the world of computer science and information technology.

To learn more about this term you can visit this Wikipedia link for more info.

Let's say that you want to convey a secret message to someone which you really don't like to tell anybody else in the classroom except that person.So how can you actually convey the message so that it can only be readable to that person and nobody else? Well here comes the term cipher, which exactly does the same as you were thinking about.But HOW?

Let's say that your secret message contains a word Hello.So what cipher does is that it actually iterates (go by) over each letter or char and adds a key to it (let's say key = 1) so H becomes I, e becomes f, l becomes m, o becomes p, so it really translates the word from Hello to Ifmmp.Sounds really hard nah? so that no one could guess that you are saying Hello to someone rather they think that you are saying Ifmmp which really doesn't makes any sense right? This is called Cipher Encryption

So this exactly how cipher works for encrypting or decrypting a text, sentence or word.