Total value locked, or TVL, is a figure used in decentralized finance to measure how much value users have deposited into a protocol. It adds up the assets locked in things like lending pools or liquidity pools, giving a rough sense of how much is being used within that application.
People often use TVL as a size indicator. A protocol with a high TVL is handling a large amount of deposited assets, which some take as a sign of activity or trust. It is a common way to compare the relative scale of different DeFi applications.
TVL has limits, though. It can rise or fall simply because asset prices change, and a high number does not by itself guarantee safety or quality. It is one data point among many, useful for context rather than as a judgement on its own.