A seed phrase encodes the master secret from which all your addresses are derived. Enter it into a compatible wallet anywhere in the world and your funds reappear, because the wallet is just software over the same keys.
That convenience is also the danger. The phrase is a bearer instrument. Anyone who reads it owns everything it controls, including future addresses you have not created yet, and every major theft campaign is ultimately a hunt for these words.
Practical rules: write it on paper or metal, store copies in separate physical locations, never photograph it, never type it into anything except the device you are restoring, and treat any person or site that asks for it, including "support", as an attacker with no exceptions.
Related: private-key, hardware-wallet, wallet-drainer, non-custodial-wallet