TVL measures the total dollar value of all assets currently deposited in a DeFi protocol - lending positions, liquidity pools, staked tokens, vault deposits. It's the most-cited DeFi metric and the easiest to misread.
TVL inflates with token price (a protocol holding ETH appears to grow when ETH appreciates without any actual user growth). TVL also double-counts: a token staked in protocol A that re-deposits into protocol B shows up in both. DefiLlama maintains the most carefully de-duplicated TVL numbers, which is why Onyx uses it as the canonical source.