What is it?
A database management system.
"MySQL is an open-source relational database management system. Its name is a combination of 'My', the name of co-founder Michael Widenius' daughter, and 'SQL', the abbreviation for Structured Query Language." Source: Wkipedia.
MySQL powers your website's database.
The Joomla! Project recommend using the latest available release of MySQL.
We recommend hosting the latest release of Joomla! on a server using MySQL 5.6 (or later).