Shamir's Secret Sharing, named after its creator, Adi Shamir, who introduced the concept in 1979, is a method for securely sharing a secret (like a private key) in a distributed manner.
The secret is divided into distinct parts, giving each participant its own unique part, with the requirement that a certain threshold must be combined to reconstruct the original secret. In the context of Bitcoin, Shamir's Secret Sharing can be used to strengthen wallet backups by splitting a recovery phrase into shares, a distinct mechanism from multisignature security.
More information on Shamir's Secret Sharing can be found by reading the official Codex32 site and watching the visual explainer below.