CMS developer or Content Management System developer is the one who’s in charge of creating websites and features using the relevant language or templates which are used in the same CMS needed.
WordPress as a CMS
One example of a CMS, is WordPress, which is used by millions of websites all around the world. With WordPress you can create and manage website with ease, even without writing code.
Plugin developer for CMS
If you are looking for a specific feature or behavior on your website, and you havent find a ready made solution for it, you probably need a CMS developer in order to create a custom plugin for your website.
Depends on your CMS and needs, the developer will use a specific programming language such as PHP for WordPress, JavaScript for most cases, MySql can be used also in order to query the database and create different plugins such as custom store filters for an E-commerce store.