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.
Now, let's say that you just got that encrypted text message lying on someone's desk and you are a really intelligent (I mean really intelligent) guy and you come to know at a glance that it is an encrypted text message which needs to be decrypted for getting it's readable form again by reversing the key which exactly someone uses to move the char (letter) to a certain position.So you guess of key = 1 and try to reverse back the letters like I to H, f to e, m to l and p to o, so it turns Ifmmp back to Hello.Ooops! the secret message is again in it's readable form again.This is called Cipher-Decryption.
So this exactly how cipher works for encrypting or decrypting a text, sentence or word.