This metric measures the cyclomatic complexity of each function in the project.

The cyclomatic complexity of a function is an indication of the number of paths that can be taken during the execution of a function. Code with many branches and loops has high cyclomatic complexity. A cyclomatic complexity above 50 should be considered bad practice and above 75 should definitely be addressed.

Functions with high cyclomatic complexity are

difficult to test since tests should be provided for each possible execution path;
difficult to understand since a developer needs to understand how all conditions interact;
difficult to maintain since many execution paths is an indication of functions that perform too many tasks.

The primary way to reduce the complexity is to extract sub-functionality into separate functions. This improves on all problems described above. If the function naturally breaks up into a sequence of operations it is preferable to extract each operation as a separate function. Even if that's not the case it is often possible to extract the body of an iteration into a separate function to reduce complexity. If the complexity can't be reduced significantly make sure that the function is properly documented and carefully tested.

M. Fowler. Refactoring. Addison-Wesley, 1999.

T. J. McCabe. A Complexity Measure. IEEE Transactions on Software Engineering, SE-2(4), December 1976.

Wikipedia: Cyclomatic complexity.