MySQLi, which stands for "MySQL Improved," is a PHP extension that allows developers to interact with MySQL databases. It provides an object-oriented interface as well as a procedural one, making it flexible for different programming styles. MySQLi supports features like prepared statements, which help prevent SQL injection attacks, and transactions, which ensure data integrity. This extension is specifically designed for use with the MySQL database management system. It offers enhanced performance and security compared to the older mysql extension, which is now deprecated. MySQLi is widely used in web applications that require database interactions.