It's tiny bit like scripting commands, which is why it's called a ScriptedTrigger, I guess.
The ScriptedTrigger will just rattle down all of its actions in the order you pur them in. It will do this instantly, except there is a delay at a certain action. Then it waits at this action until the delay is over and then it continues. The following three are the most common delays, I think:
WAITFORTIMER means it waits for a timer to count to zero. You can set the number it should start from in the field then. E.g. if you set it to 10 the ScriptedTrigger will pause for 10 seconds once it gets to this action and after those ten seconds it will rattle on from the next action.
WAITFOREVENT means it waits at this action until the event you set in the field is triggered somewhere. For example by a regular trigger. Once it detects this event being fired somwhere it goes on to the next action.
WAITFORACTION means it waits until a certain action withing the ScriptedTrigger is reached. All the actions are numbered starting with 0.
For example, I could have a ScriptedTrigger that has 7 actions which are all necessary for a specific task (e.g. waiting for someone to press a button someplace and then blowing stuff up with special effects and all) and another couple of actions after that that deal with the next objective. Actions number 0, 1, 2 and 3 are all responsible for making sure the player does everything that is necessary and then they make the blocking object move out of the way. 4 and 5 are responsible for explosion effects when this happens and 6 and 7 are responsible for after-math effects, like scarring projectors or lingering smoke, etc.
Now, I could want the ScriptedTrigger to commence at action 9 as soon as the way is free (because the lingering smoke that follows after a delay isn't important for the gameplay), so I can set action 8 to WAITFORACTION 3. As soon as action 3 is reached, action 9 starts too then.
GOTOACTION is similar, but it lets you jump to a certain action. E.g. if I have 4 actions. Action number 0 is WAITFOREVENT and number 2 is for an explosion and 3 is a delay of 10 seconds (WAITFORTIMER 10). A player can press a button somewhere and thus trigger and event. The ScriptedTrigger picks this up and starts. The explosion happens. Then I want the player to be able to do the same thing again after 10 seconds have passed (e.g. time for the explosive to rearm itself. Maybe even with sound!?). So I make action number 4 GOTOACTION 0. The ScriptedTrigger then jumps back to action 0 and the trap is set again.
I hope this helps a bit.
Here is some more info you might find helpful:ScriptedTrigger page at the Unreal Wiki