Referred to as the MySQLi extension or MySQL improved extension. Allows access to additional features provided in MySQL 4.1.3 and newer. It is included with PHP version 5 and later.
The MySQLi is an extension that opens up the ability to take advantage of new features found in MySQL system version 4.1.3 and newer.
Documentation from MySQL provides a detailed overview of the extension. there are many enhancements and benefits to using this extension over the previous MySQL extension. If you are running MySQL 4.1.3 or higher it is highly encouraged to transition or begin using the MySQLi extension.
Additional information can be found in the following link: MySQLi Extension Function Summary
This tag should be on questions to show that you are utilizing this extension. Additional tags that might be seen combined with this tag would be php and/or the mysql tag or the specific version of MySQL you are working with such as mysql-5.5