The conditions fields appear in a Trigger screen when the Apply Conditions field is enabled (checkmark visible), or they may be used as a standalone trigger. However, do not be confused by Event Trigger Conditions, as shown below, and Condition Triggers. Event Trigger Conditions are conditions added to an event trigger - for example, if the trigger is absolute time such as 9:00PM, then an Event Trigger Condition can be added that "time is nighttime" so that the event only triggers if 9:00PM is after sunset. Condition Triggers are when you use conditions as the trigger. Conditions, when used as the trigger only, are evaluated approximately once per minute. The reason for this is very simple - if your condition is that the date is April 1, 2006 and today is April 1, 2006, then the event would trigger repeatedly at a very rapid rate, for the entire day, if conditions were evaluated as often as possible. It is recommended that if there is a trigger that would work, then use that and add conditions - conditions as a trigger is a last resort.
For more information on conditions as a trigger, please refer to this page.
Trigger conditions can be required or optional. Required conditions are separated by "AND" in the Events screen and optional conditions are separated by "OR". There can be multiple required or optional conditions for each event. The required conditions are set in the first grouping of fields in the screen below and the optional conditions are set in the second grouping of fields.
Click here for more information on how event conditions work.
To learn more about the different types of conditions and how they are added or removed, click here.