BIP39 means Bitcoin Improvement Proposal 39. The words are selected from a selected list of 2048 common English words which can be used to the generate a binary seed (your private key). This extra complexity increases the threat of misplacing the passphrase, and so shedding your copyright. From this issue https://reallivesocial.com/story5053553/the-greatest-guide-to-seed-phrase-bip39