The Vigenere cipher is a polyalphabetic cipher that was invented in the 16th century. This method of encryption uses multiple of Caesar ciphers with a keyword to switch the key every letter rather than a single key, making it harder to decrypt. As it is polyalphabetic cipher, it is not vulnerable to regular decryption cracking methods such as frequency analysis because multiple letters can be represented the same letter.
It works by getting the value of each letter in the keyword and using that as the amount to shift for the letter in the message. For example, if the message was “HELP” and the keyword was “abc”, then the Caesar cipher key values would be 0,1,2,0. When the key runs out of characters it loops back to the start of the keyword. The encrypted message would be “HFNP”.