A CMS, short for content management system, is software you use to create, edit and publish the content of a website without writing code yourself. You type a text, add an image and click publish; the CMS turns it into a page. Well-known examples are WordPress and, for online shops, Shopify and WooCommerce.
Why does it matter?
The CMS largely determines what HTML your website returns, and therefore what search engines and AI crawlers read. A CMS already takes care of many technical matters: Google mentions, for example, that a CMS such as WordPress often creates a sitemap itself. Other things depend on the theme, the settings and the plugins or apps you install, such as canonicals, structured data and the structure of your URLs.
Example
Say an online shop installs two plugins that both add product data as JSON-LD. The admin screen shows nothing of this, but the HTML contains two conflicting descriptions of the same product.
Common mistake
Assuming that what you see in the admin screen ends up in the HTML the same way. Check the page source. That is why Atlas, the research engine developed by Outseen, investigates the HTML the website actually returns, not the templates in the CMS.
Outseen has a standard approach for online shops on Shopify and WooCommerce; we discuss other systems beforehand. A comparison is in Shopify or WooCommerce: which suits your online shop?