Test coverage is a measure of how much of your code is actually exercised by automated tests, usually reported as a percentage. If your tests run 800 of 1,000 lines, you have 80 percent coverage, and the other 200 lines are running in production without any test watching them.
A helpful comparison is a building inspection. Coverage tells you what share of the rooms the inspector actually walked through. Eighty percent checked is reassuring, but it does not prove the inspected rooms were examined thoroughly, and the unchecked twenty percent is exactly where a nasty surprise can hide. Coverage works the same way: it shows where tests reach, not how good those tests are. An inspector can stroll through a room without opening a single cupboard. A test can run a line of code without ever asserting that the line did the right thing. So a high number feels safe and can still be hollow.
It is closely tied to your unit tests and the test cases behind them. Used well, coverage is a guide that points to risky, untested areas so a team can decide what deserves more attention. Used badly, it becomes a number to game, where people write shallow tests just to push the percentage up. The trick is to treat the gaps as information, not as a score to beat. A report that shows the payment logic is barely tested tells you exactly where to spend the next hour, and that is worth far more than a green badge on the readme.
At TopDevs we treat test coverage as a guide rather than a goal, focusing tests on the parts of a client’s software where a failure would hurt most, instead of inflating a percentage that proves nothing.
