CMS stands for Content Management System. It is a program/interface that allows to publish, edit and delete content from a website. CMS is also know as the Back-office (which is the part of the website that only the ADMIN has access to) vs. the Front-office, which is the website itself that all internet users can visit.
A CMS should first of all be user-friendly and allow you to complete all kinds of actions by yourself regardless of your professional background, IT related or not (such as uploading pictures, creating sliders, adding events, creating and sending newsltetters, publishing your latest news…).
Always ask your developer what kind of CMS he’ll be using when developing your website. The top 3 CMS in my opinion are: