A recovery seed is explained in full under seed phrase, which covers how the words are generated, the standards behind them and how wallets derive keys from them. This page explains the alternative name, which many hardware wallet makers and exchange help pages use.
A recovery seed is a list of ordinary words, usually 12 or 24, shown to a user when they create a self-custody wallet. The words encode the random number from which every private key in the wallet is derived, so anyone holding them can rebuild the wallet on another device and spend its funds. In a hierarchical deterministic wallet, one seed can generate a practically unlimited number of addresses.
The term "recovery" stresses the purpose: if a phone is lost or a hardware wallet breaks, the seed is how the owner gets access back. Other names for the same thing include recovery phrase, backup phrase, secret recovery phrase and mnemonic phrase.
Because the seed is the wallet, guidance is consistent: write it down offline, store copies somewhere safe from fire and theft, and never type it into a website or share it with anyone claiming to offer support. Many thefts start with fake apps or messages asking for the recovery seed, a common form of phishing.