Hierarchical Deterministic (HD) Wallets
An HD wallet derives an unlimited tree of keys and addresses from a single seed in a predictable way.
In depth
"Deterministic" means the same seed always produces the same keys, so one backup covers everything forever. "Hierarchical" means keys are organised in branches — one for Bitcoin, one for Ethereum, sub-branches for accounts. This is the standard almost every modern wallet uses.
Why it matters
It is why restoring a seed phrase in a new wallet app brings back all your addresses and balances.