Test coverage is a measure of how much of a project is exercised when its tests run. It is like a map showing which rooms have been visited during an inspection.
A report might show that tests ran through many lines of code, while some parts were never reached. This can help a team notice areas that need more checks.
A high coverage number does not guarantee good tests or a bug-free app. The tests still need to check meaningful outcomes, not simply touch many lines.