BIP39 means Bitcoin Enhancement Proposal 39. The words are chosen from a certain list of 2048 widespread English words which may be used to the deliver a binary seed (your non-public key). Standard recovery phrases use a particular list of 2048 words pursuing the BIP39 standard (Bitcoin Improvement Proposal 39). https://webcastlist.com/story19962145/a-secret-weapon-for-bip39-words-to-numbers