A BIP39 mnemonic sentence is a set of words (most commonly 12 or 24) that we can interact with more successfully than other characters that aren't words (bits and numbers and the like). BIP39's purpose in introducing the use of mnemonic words is to reduce the errors that often occur when we try to communicate long strings of unfamiliar characters. The wallet handles the heavy lifting of converting the mnemonic words through a process of complex hashing, salting, and checksum operations into a binary "seed," which is then used to generate the all-important private and public encryption keys. It's very important that the mnemonic sentences are generated from randomly selected numbers! These numbers can be generated in a variety of different ways, which are described in other articles here, like "What is an RNG" and Seed Phrase Generation.