BIP39 means Bitcoin Improvement Proposal 39. The words are picked from a certain list of 2048 widespread English words which may be accustomed to the generate a binary seed (your private key). Standard recovery phrases use a certain list of 2048 words pursuing the BIP39 standard (Bitcoin Advancement Proposal 39). https://getsocialsource.com/story4194906/a-review-of-bip39