CMS stand for Content Management System. The most common use of a CMS is building websites and web applications with them. They are super useful tools as they really do what the tin says: they make managing web content easy without you having to build a website from scratch. You get a frame of a website on which you choose and install a theme and all there is left for you to do is fill it with text and photos.
The very good thing about CMSs is that they can be tweaked to look or act in a certain way. Some features can be switched on and off by the user but if you want a bigger change you will need somebody who can tweak the code for you. Altering the code is made easy by whichever CMS you use so a developer can go a long way to get you exactly what you want.