A seed phrase (also called a recovery phrase or mnemonic) is a human-readable representation of the master entropy that generates a wallet's private keys. The standard (BIP-39) encodes 128 or 256 bits of entropy as 12 or 24 words from a 2048-word dictionary. From the seed, a hierarchical deterministic (HD) wallet can derive an unlimited tree of addresses, all backed up by the single phrase.
When you create a wallet, the wallet software generates random entropy, runs it through PBKDF2 to derive a seed, then generates a hierarchy of keys per BIP-32. You write down the words. As long as you have the words, you can restore the wallet on any compatible software, on any device, forever.
The dual nature of seed phrases is the security model: anyone with the phrase has full control of the wallet, and there is no recovery if it is lost. No password reset, no support team, no fraud reversal. Storage practices (metal backups, geographic distribution, family duress procedures) are an entire field.
Seed phrases are the foundation of self-custody. They are also the largest single attack surface in crypto. Phishing, malware, and social engineering for seed phrases is a multi-billion-dollar criminal industry.
The cold storage entry covers how to keep seed phrases safe long-term. The API security article covers parallel concepts for API keys.