This is a pseudo column available in the SELECT query. Generates a running row number something like the window function "ROW_NUMBER() OVER()" This can be used in the select clause to list the running row numbers or used in the WHERE clause to limit the output rows just like in MariaDB LIMIT.

Oracle: SELECT * FROM TAB WHERE ROWNUM <= 10;
MariaDB: SELECT * FROM TAB LIMIT 10;

Oracle Doc: https://docs.oracle.com/cd/B28359_01/server.111/b28286/pseudocolumns009.htm#SQLRF00255