MySQL is a high performance scalable database to store all of your website info

Formal description:

MySQL - one of the most popular free database management systems on the Internet.

In other words:

MySQL - offers good performance, reliability and flexibility. PHP code is written to connect the website to the MySQL database, then the database running MySQL stores all the site's information on its server. Working with MySQL, as a rule, can be rendered without much difficulty. 

What is it used for?

Long-term storage of information. When working with files, we place information within a number of lines to be saved and retrieved later for further tweaking. The task of long-term storage of information is commonly needed in programming web-based applications. Commonly stored information include; recording in the counter, leaving messages in the forum, remotely storing content on the site, etc.

Professional techniques for storing files are very complicated. Take for instance, the additional actions needed when inputting information, sorting said information, and finally retrieving this information. All these actions would be performed on the server's host provider where, very likely, there is only one variant of Unix, hence the need to also take extra care of the rights to accessing files and the file's placement. Using this technique, the amount of code increases significantly and making a mistake in the program is very common.

All these problems are solved by using a database, like MySQL. Databases tend to the safety of information, its sorting and allow you to retrieve and post information through a single line. Coding, when going through a database, is a more compact, and is therefore, much easier to debug. Lastly, the speed of retrieving information from the database is much faster.