CMS stands for content management system. It is the admin area behind a website where you log in to add pages, update prices or swap photos, a bit like editing a document.
WordPress is by far the most widely used, but Wix, Squarespace, Shopify and Webflow all include one. Some modern websites do not use a traditional CMS at all, and changes are made by the developer instead.
A CMS is handy if you want to make regular changes yourself. It also brings upkeep: updates, security patches and plugins that need looking after.
Ask yourself honestly whether you will edit the website. If you will, a CMS is useful. If you would rather text someone ‘please change the Saturday hours’ and have it done, you may not need one, and you avoid a system that has to be updated or it becomes a security risk.