BIP39 means Bitcoin Advancement Proposal 39. The words are picked from a particular list of 2048 prevalent English words which can be used to the produce a binary seed (your personal crucial). Standard recovery phrases use a particular list of 2048 words following the BIP39 standard (Bitcoin Enhancement Proposal 39). http://rylanmecp341.wpsuo.com/from-words-to-wide-range-exactly-how-to-efficiently-make-use-of-bip39-passphrases-for-your-copyright-pocketbooks