Knowledge of global variables principally allows the alteration of various setings that would otherwise require a restart of mysql.

To list current variables:

show variables;

To list current variables with 'slow' anywhere in the name:

show variables like '%slow%';

To set a global variables, this example to increase max_connect_errors:

set global max_connect_errors = 20;
Query OK, 0 rows affected (0.00 sec)