The short answer
The closing line is the final price a market trades at before it closes. It has absorbed everything the market learned: team news, weather, injuries, and all the money bet along the way.
If you took 2.10 on something that closed at 1.90, you beat the closing line. If you took 1.85 on something that closed at 2.00, you did not.
Beating the closing line consistently means you are finding information or errors before the market corrects them. That is a statement about your process, and it arrives long before your results can say anything reliable.
Why it reads results earlier than a win rate does
A small edge applied to a small number of bets is completely buried in variance. A bettor with a genuine edge can lose over hundreds of bets, and a bettor with no edge can win over hundreds. Neither run proves anything.
Closing line value has far less noise in it, because it does not depend on how any event turned out. Every bet gives you a reading immediately, and the reading is about the price rather than the result.
This is why sharp bookmakers pay attention to it too. A customer who consistently beats the close is identifiable long before that customer has won any meaningful amount, which is part of why accounts get restricted early.
A worked example
Suppose you back a team at 2.20 and the market closes at 2.00.
Convert both to implied probability. 1 / 2.20 is 45.5 per cent and 1 / 2.00 is 50 per cent. You took a price implying 45.5 per cent on something the market finished believing was a 50 per cent chance.
The gap is 4.5 percentage points in your favour, before accounting for the margin in either number. Whether that particular bet wins or loses does not change the fact that you got a better price than the market's final assessment.
What closing line value does not tell you
It does not mean a bet was good. You can beat the close on a bet you should never have made, and you can lose to the close on a sound one. Over one bet it is noise.
It does not work everywhere. The logic depends on the closing price being informative, which requires a market with enough volume and attention to actually converge on something. In thin markets the close is not much wiser than the open.
It is not a projection of anything. Beating the close consistently is evidence that a process is finding something. It is not a forecast of results, and nothing on this page suggests what anyone might make.
Comparing against the wrong closing price also breaks it. The close at the book you bet with is the honest comparison; using a sharper book's close flatters your own numbers.
How to use it in practice
Record the price you took and the price at close, for every bet, from the moment you start. Reconstructing it later is not possible, because the closing price is gone once the event begins.
Convert both to implied probabilities before comparing, so markets with different margins are comparable.
Read it over a large sample and as a trend rather than bet by bet. A single result says nothing; a persistent pattern across hundreds of bets says a great deal about whether your process is doing anything.