Counter is the act of stopping a boss from casting his next ability. This can be done by using 16px , or . To counter a bosses' ability, one of the classes with the named spells need to cast their countering spell on the boss before the timer of the ability runs out. The timer beside the ability name above the head of the boss is an indication that the ability is counterable.
Each boss has many counterable spells but not all of them are as dangerous as the others so it's important to organize your counter order to make full use of your counters.