BIP39 means Bitcoin Improvement Proposal 39. The words are chosen from a specific list of 2048 typical English words that may be used to the make a binary seed (your non-public key). Standard recovery phrases use a certain list of 2048 words subsequent the BIP39 standard (Bitcoin Advancement Proposal 39). http://josuejnub341.image-perth.org/from-words-to-riches-exactly-how-to-effectively-use-bip39-passphrases-for-your-copyright-wallets