An Acronym is a word formed by using one or more letters of the words in a phrase. It’s used as an abbreviation of that phrase, e.g. USA, TEFL, NATO.
A Backronym is when we take an existing word and invent a phrase around those letters. It is, if you like, formed in the opposite way to an acronym, e.g.TEFAL = Teaching English For A Laugh.
For more, see Acronyms & Backronyms in English.