Your seed phrase (or recovery phrase) is a list of 12–24 words that can restore your entire wallet. Anyone who has it can take your Bitcoin, and if you lose it with no backup, your coins are gone forever. It's the single most important thing to protect.
Do: write it on paper (or stamp it into metal) and store it offline in a safe place — ideally more than one location. Consider a fireproof and waterproof backup for larger holdings.
Don't: never type it into a website, never store it as a photo or in cloud notes, and never share it with "support," giveaways, or anyone who asks. No legitimate service will ever need your seed phrase.