The best answer for you should be this
https://coinmarketcap.com/alexandria/glossary/total-value-locked-tvlIn my understand if TVL was total assets that got locked into the defi platform. This assets represent total assets that brought by all of defi users into the platforms. These assets being staked on the defi platform to gain the interest like staking yield or farming yield.
You can try to read the article that i have mentioned above to know more about that. It means how much assets that were holding by a defi platform. it can also be used to determine how big the user base of such platform.