Effect triggers are on the first column of the effect builder, it tells you when the effect runs.
For example, matchInit triggers the effect when the match starts, healed triggers the effect when the card is healed, and an effect trigger for damaged tells the card to run the effect when the card is damaged.
Most game templates have an extensive preset list of effect triggers for your cards to respond to. Please refer to the individual game templates wiki for the full explanation.
Creating your own triggers
The template triggers are great at getting started, but if you want to do cool new effects that weren't already thought of by Dulst, you should create your own triggers.
Effect triggers are just the target of a "run effect" action. When a run effect action target cards and target specific triggers, the line with the effect trigger selected will run. Typically, you would place the run effects inside rule cards that control the game logic and call the triggers at the appropriate time. For example inside the "deal damage" rule card we have a run effect action calling to run the "damaged" trigger.
This ensures that the target card will run the "damaged" line, the line marked by the effect trigger "damaged", after the damage has been dealt by the deal damage rule.
After setting the run effect line, you can use the trigger in the effect trigger column. If it's not in the list, you can put it into the "custom" field.