A secret key, in the sense used by crypto wallets, is explained in full under private key. The two phrases are often used interchangeably, and some wallets and documentation say "secret key" where others say "private key". This page explains the overlap and where the meanings diverge.
In public-key cryptography, each user has a key pair. The public key, from which a blockchain address is derived, can be shared freely. The matching secret or private key must never be revealed, because whoever holds it can sign transactions and move the funds. Wallets usually derive many private keys from a single seed phrase, which some apps also call a secret recovery phrase.
Outside wallets, "secret key" has other meanings that are easy to confuse. In symmetric encryption, a single shared secret key both locks and unlocks data, with no public half. Exchanges and payment services also issue an "API secret key" alongside a public API key, which authorises automated access to an account. Leaking that kind of key can also lead to losses, so services allow it to be restricted to read-only or trading permissions. In every case, the rule is the same: anything called secret should stay offline, out of screenshots and away from anyone who asks for it, including callers posing as support staff.