Codacy uses cyclomatic complexity to identify complex files in your repository.
Cyclomatic Complexity is a software metric used to indicate the complexity of a program. It is a quantitative measure of the number of linearly independent paths through a program's source code.
A complex file is a file with a complexity value bigger than the threshold defined on the repository quality settings.
Code complexity is calculated based on the maximum number of complex methods in a file.
In your Repository dashboard you can see how complexity is evolving for your repository.