Vigenère invented an encryption method that combines several additive ciphers. In modern terminology we could say that the key is a string of numbers, each one between 0 and 25. The easiest way to remember this is to convert it into a word,  using A = 0,B = 1,.... For example, the key 1,17,14,22,13 is equivalent to the keyword BROWN. Then the plaintext is written out, with the keyword underneath it, repeated as necessary. For example, to encrypt the message the computer has become very useful we write

                               THECOMPUTERHASBECOMEVERYUSEFUL

                              BROWNBROWNBROWNBROWNBROWNBROWN

Now each letter is encrypted using the additive cipher whose key is below it. In the example, T would become U, H would become Y, and so on. The message becomes

                         UYS YBNGIPRS YOO OFTCIR WVFU HTVTQY.

In fact, the ciphertext would not usually be broken into words. You would probably receive

                           UYSYBNGIPRSYOOOFTCIRWVFUHTVTQY,

or perhaps it would arrive arbitrarily broken into four- or five-letter groups in order to make copying errors less likely:

                             UYSY BNGI PRSY OOOF TCIR WVFU HTVT QYZZ