Plain meaning
Start with the shortest useful explanation before going deeper.
A deprecated Solana on-chain automation protocol (by Clockwork Labs, now sunset) that allowed developers to schedule and trigger smart contract instructions based on time (cron expressions) or on-chain conditions using a network of automation worker threads running alongside validators. Clockwork introduced the concept of Thread accounts that encoded trigger conditions and instruction targets directly on-chain, and while it has been discontinued, it pioneered the automation primitive model later explored by projects like Turbin3 and off-chain keeper bot architectures.