BIP39 means Bitcoin Enhancement Proposal 39. The words are selected from a selected list of 2048 common English words that may be used to the deliver a binary seed (your personal important). This extra complexity boosts the chance of misplacing the passphrase, and so dropping your copyright. Introduced in 2013, https://singnalsocial.com/story3978305/facts-about-bip39-seed-phrase-github-revealed