Explanation
A Vigenère cipher is similar to a Caesar Cipher but uses a keyword to determine the offset of each letter. Each letter in the keyword corresponds with a letter in the message and determines how much it should be offset by. For example, if the first letter of the keyword is A, that means the first letter of the message will be offset by 0. If it is offset by B, the first letter will be offset by 1.
If the keyword is shorter than the message being ciphered then the keyword will be repeated until it is of the same length.