A coverage tool records which lines, branches or functions execute during a test run. The report can show which parts of a codebase still lack tests that reach them.
A high coverage percentage does not show whether tests check the right outcomes. A test can execute a branch without detecting an incorrect result, so review the assertions as well.
ELI5
Test coverage shows how much of a program the checks actually try out.
Imagine checking the lights in a house. If you try only the downstairs switches, the upstairs lights are still untested.
Does full coverage prove a program works?
No. A test may run code without checking the right result, and a coverage tool cannot account for every possible input.
Which kinds of coverage can you measure?
Common reports count executed lines, functions or branches. Each describes a different part of the code that the tests exercised.
