MariaDB is a database management system (DBMS), which is based on MySQL and is largely compatible with it.
MariaDB and MySQL are fully compatible with the syntax of SQL queries. That is if your program uses MySQL databases (for example, a website in PHP), then when you upgrade to MariaDB, you do not need to change anything in the program.
MariaDB is also binary compatible with MySQL connectors. Therefore, you do not need to change them after upgrading.