xdebug.mode = develop ​

This mode enables the development helpers of Xdebug and shows for example a better formatted var_dump() output or a more detailed Stacktrace. Since many PHP frameworks, like Symfony, have their own development tools included, this doesn´t get used much anymore. These tools are usually available once you put the site into "dev" mode where you can get a detailed stacktrace or special commands like dump() or dd which prettify the var_dump() output.

If you´re not using a framework, or using one without that functionality, you can find out more about this mode in the official documentation.