BIP39 means Bitcoin Improvement Proposal 39. The words are selected from a certain list of 2048 widespread English words which can be used to the generate a binary seed (your private key). This article points out BIP39, the standard that defines how seed phrases are made, and summarizes how BIP32 https://sergiorgzi464.weebly.com/blog/from-words-to-wealth-exactly-how-to-properly-use-bip39-passphrases-for-your-copyright-budgets