A seed phrase is an ordered set of 12-24 words that is generated by your wallet and stores all the information that can be used to recover your wallet. For the most part, seed phrases follow Bitcoin Improvement Proposal: 39 (BIP 39). BIP 39 standard describes the process in how a mnemonic phrase (a group of easy to remember words) is generated and can be used to recover your wallet in the event your wallet fails.
Inheritance. For most of us, it's not a priority but it will be a concern eventually. We want to ensure our loved ones are taken care of after we kick the bucket. It's somewhat straight forward with fiat money and common assets (home, investments, property, etc). But with cryptocurrency, it's a bit less straightforward. It is financial self custody. No one is holding your money but you. Third parties are non-existent (unless you're actually using a custodial service to hold or manage your coins).
Lately, we've heard of people generating a new seed phrase for every new wallet they use. By "wallet", we are referring to the "brand" name of wallet software or hardware. If this is you, you don't have to do this. Your seed phrase (or your coins for that matter) isn't tied to a specific wallet.