CMS is short for Content Management System is a back-end function for users to update what they want to put onto their website. E.g. Update their promotions panel. Depending on sophistication of the CMS, you may or may not want to hire a freelancer.
Most freelancers when doing CMS, will be of a popular open source CMS application as shown below. It allows the user to update simple content such as blog posts. But the functions may stop there, depending of which kind of page builder they used for your template. Also, many users of open source CMS complained the difficulty of navigating through this as there are many tabs and functions.
Website Design companies will create custom CMS from scratch to suit your needs. For example, if you need a gallery, news panel, forms and posters. They will create a backend system to cater to it. This is to improve user experience and make it less confusing to navigate the backend system. Below is an example of a backend system that Elves Lab designed.
(An example of a messy CMS)