TL;DR
A seed phrase (recovery phrase) is a sequence of 12 or 24 words that serves as the master key to your crypto wallet — anyone who has it controls all funds in that wallet.
When you create a wallet (Phantom, Solflare, etc.), it generates a random seed phrase and derives your private key from it. The seed phrase is a human-readable representation of the entropy that creates your wallet. From a single seed phrase, the wallet can derive multiple accounts and keypairs using a derivation path. This is why the same seed phrase restores all your accounts when you set up a new device.
Your seed phrase IS your wallet. Anyone who has it can restore your wallet on any device and take everything. Unlike a password, it cannot be changed or reset — if compromised, you must create a new wallet and move your funds immediately. No legitimate service, developer, support team, or airdrop will ever ask for your seed phrase. If anyone asks for it, it’s a scam, 100% of the time.
Write it on paper or stamp it in metal and store in a secure physical location. Never store it digitally — not in notes apps, photos, cloud storage, email, or password managers connected to the internet. Never enter it on any website. Consider splitting it across multiple secure locations. For large holdings, use a hardware wallet where the seed phrase never touches an internet-connected device.