MySQL is a database management system, it is open source and uses structured query language (SQL) to work with database content. Open source code makes it very popular among software developers, which explains its frequent use in various projects.
The most modern CMS (content management systems), such as WordPress, work with it as a data warehouse. All settings and changes you make on the site are entered into a database running MySQL, and when, for example, a new visitor is registered on your site, the contents of the database are processed.