How does test(pkg; coverage=true) work?

I tried setting the coverage option to true. It generated *.cov files in the tests folder of one of my packages, which contain numbers on the left margin, but what do they mean? The documentation is not very explicit.

I think it counts the times a specific line of code was executed. See

$ julia --help | grep coverage -A 1
 --code-coverage={none|user|all}, --code-coverage
                           Count executions of source lines (omitting setting is equivalent to "user")