BIP39 means Bitcoin Advancement Proposal 39. The words are chosen from a certain list of 2048 widespread English words which may be used to the produce 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://writeablog.net/ipennywpzc/h1-b-opening-the-power-of-bip39-exactly-how-to-produce-and-use-mnemonic