This metric measures the total cyclomatic complexity for the functions in a file.

Cyclomatic complexity approximates the number of paths that can be taken during the execution of a function (and hence, the minimum number of tests cases necessary to test it thoroughly). Straight-line code has zero cyclomatic complexity, while branches and loops increase cyclomatic complexity.

Files that contain too many complex functions can be difficult to test, understand, and maintain.

Try to simplify overly-complex code. For example:

Highly nested conditionals can be simplified by rethinking the requirements that the function fulfills.
Repeated tests can be refactored into helper functions, which also decreases the risk of introducing defects by copying and pasting code.
Large complex functions can often be split into smaller more focused functions.

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.