Backdrop CMS is written in the PHP programming language.
That means if you want or need to extend the functionality of Backdrop with custom modules and/or themes then you'll need to know how to write code in PHP.
To get started with PHP and then Backdrop CMS development you'll need a place to write your PHP code. That could be a webserver or a local development environment. For this series we'll use the local development environment called Lando.
Once you have lando installed you can start writing PHP by itself or for backdrop quite easily.