BIP39 is a standard that defines how a recovery phrase is generated from a list of 2,048 words and how it maps to a wallet's seed.
A random number is generated, split with a checksum, and encoded into words from a fixed wordlist. The result is a 12 to 24 word phrase you can write down.
Private keys are long and hard to record by hand. BIP39 turns them into memorable words that are easy to write and verify, reducing transcription errors.
Because BIP39 is a standard, the same phrase can restore the same accounts across many different wallets that support it.
The phrase is your wallet. Anyone with the phrase controls the funds, so protecting it is protecting your crypto.