A CMS is a dynamic website where multiple users can edit, update and maintain the content. They are simple to learn and easy to set up. One of the most popular examples of a CMS is WordPress.
In the term of SEO, it is a bit of a trade-off – a website can be set up in a matter of minutes using templates, but you do not have any control over the code. So once the template is in use, you will not be able to make any changes. On the other hand, this is ideal for people that do not have a desire to deal with code. There are plenty of SEO plugins you can use to help with SEO activities.