A seed phrase, also known as a recovery phrase or backup phrase, is a list of words that store all the information needed to recover a Bitcoin wallet. Seed phrases are an easy way to backup and restore a wallet. A Bitcoin seed phrase consists of a random selection of twelve to twenty four words from a given wordlist.
It contains 2048 mnemonic words from the BIP39 list, so each word in the seed phrase has 1 in 2048 chance of being selected. This makes it difficult for anyone to guess what your seed phrase is, even if they know some of the words. The words are usually written in a seemingly random order, making them hard to remember as well.
A seed phrase can be used to generate a master private key (also known as a root key or mnemonic seed) which can be used to derive child keys (e.g. addresses). Seed phrases are often used to restore cryptocurrency wallets because they are easier to store securely than maintaining a string of characters in the form of a private key.
They also make it easier to share the wallet with an inheritor, since many people are more comfortable sharing words than long strings of characters.