MySQL is a database system . It is used in several platforms (Windows, Linux) and programming languages.
MySQL has been released as open source under the freely available GNU General Public License.
How WordPress uses MySQL
WordPress uses MySQL to load and store data. Currently, mariaDB is also becoming popular.
MySQL database stores everything you put on your WordPress page: created articles, tags, categories, authors, user data and the like. When a visitor to your website loads its content, the WordPress theme and plugins communicate with their databases. They will provide them with the required information. The information will then be displayed on the website.