This metric measures the maximum nesting depth of statements. When statements are nested too deeply in the code they become difficult to understand. This is because understanding a line that is deeply nested requires an understanding of the context of that line.

The best way to address this problem is to extract more deeply nested parts of the code into their own logical methods.

This method has a nesting depth of 4.

This code is easier to read because the part that prints every character in the string along with its character code has been extracted to a separate method.

Martin Fowler. Refactoring. pp. 89-95. Addison-Wesley. 1999.

Steve McConnell - Code Complete: A Practical Handbook of Software Construction.