A seed phrase is one way to generate a private key. It is a collection of words – such as: school, bus, towel, rose, bread – called a “mnemonic.” By saving these words and entering them into special software, the software can generate a private key to create a wallet or recover that same private key and wallet later.
The software is needed because private keys are often very long strings of text and numbers. Writing them down or storing them is both tedious and error-prone. The slightest mistake, for example, the misplaced capitalization of a letter or an “L” disguised as an “i” or a “1”, could mean that funds are irreparably lost.
Some of the more popular seed phrase technologies used across software and hardware wallets are BIP39 and SLIP39.