Trigger Effect

Source: Yugioh Wikia

Trigger Effects activate whenever a certain condition on the Field, the nature of play, or the progress of the turn, is met.

The effect of "Zaborg the Thunder Monarch" is triggered when it is Tribute Summoned, and the effect requires it to destroy 1 monster on the field.

Optional Trigger Effects are effects that don't happen automatically when the trigger condition is fulfilled, the controlling player decides whether to activate it or not. These effects involve cards like "Mystic Tomato", "Peten the Dark Clown", or "Dark Magician of Chaos". With "When" Optional Trigger Effects it is possible to Miss the Timing.

When two Optional Trigger Effects try to activate at the same time such as two copies of "Mystic Tomato" destroy each other in battle, neither will miss the timing, as the last thing to happen was both being sent to the Graveyard. Both of the effects trigger at the same time, and form a Chain. The Turn Player's effect becomes Chain Link 1, and the opponent's effect becomes Chain Link 2.

Only the activation of an Optional Trigger Effect can miss the timing. In other words, as long as the effect is activated, the resolution will never miss the timing. So two monsters will be summoned in the above example

Compulsory Trigger Effects are effects that must resolve, and are not Optional. When Simultaneous Effects Go On Chain, and two or more effects trigger at the same time, the Compulsory effects all go to the beginning of the Chain, and the Optional effects are put to the end of the Chain.