Those are just plain English. Line coverage (how many lines in your source code are covered by unit test cases) is just 17%, while branch coverage (how many if-else branches are covered in unit test cases) is just 18%.
While 100% coverage is rather difficult to achieve (you need a lot of refactoring to decouple hard-to-cover items), you usually can go above 50% and even 80% by adding more test cases, which can be good enough.