A static website is a collection of webpages that does not need to change often.
An example of this would be the website of a construction company, that would display contact information and feature a few completed projects.
A web designer would need to make updates for the client, which would be associated with a cost for each update.
A CMS is a website that the client can update with ease.
With this technology you don't have to rely on a web designer to change simple things like upcoming events, contact information, staff names, times that change and whatever else might need to be tweaked.
Create or edit webpages from anywhere. Just as easily as sending an email, really.