BIP39 is the technology that describes how a cryptocurrency wallet creates a series of words (or "mnemonic code") that make up a mnemonic sentence, which the wallet turns into a binary "seed" to create encryption keys and then uses that to execute cryptocurrency transactions.
Bitcoin Improvement Proposals, or BIPs, are essentially design features that allow for input, ideas, information, or changes to the way Bitcoin works.
One of these features is BIP39 (short for Bitcoin Improvement Proposal 39), a standard that proposes the use of a mnemonic phrase (a group of easy-to-remember words) as a backup to recover your wallet and coins if your wallet is lost or destroyed. This is also known as a seed phrase, recovery phrase, wallet backup, etc.
The wallet will generate this phrase for you when you create a new wallet, if you have an existing phrase you can regenerate your wallet by entering it into the wallet.
When you create a new wallet, you will typically be asked to write down a 12-24 word phrase that will serve as a recovery method if your wallet becomes corrupted or stops working.