BIP39 or Bitcoin Improvement Proposal: 39 is one of the many design ideas that was approved by an economic majority of the Bitcoin community and became a standard for many popular wallets.
BIP39 is the use of a mnemonic phrase -- a group of easy to remember words -- to serve as a back up to recover your wallet and coins in the event your wallet becomes compromised, lost, or destroyed.
This is also known as a mnemonic seed (phrase), recovery seed (phrase), wallet back up, master seed, etc.
These words aren't just any words. They are pulled from a specific list of 2048 words known as the BIP39 word list. Upon start up, wallets that utilize the BIP39 standard will provide you a 12-24 word phrase randomly chosen from the standard BIP39 wordlist.