Drupal has a number of coding standards and best practices that govern the way code should be written. This has many benefits but can allow for a consistent and maintainable code to be created.
All Drupal modules and themes are written with coding standards in mind and that allows Drupal developers to look at any project and see a similar style of code. If you ever submit code to Drupal or a contributed project you will be required to adhere to these coding standards, so it makes sense to get used to knowing about them.
The main tool that is used to test coding standards in Drupal is PHP_CodeSniffer, which is a widely used static analysis tool for testing PHP coding standards. It comes with a set of coding standards built in, but it is possible to add the Drupal coding standards for the tool to use.