Actions are basically things that happen inside events, you specify actions inside the events and they specify what should happen for the event when it is run.
There are multiple types of actions, and an event can have unlimited actions, in the Core Body section you have already looked at an example of implementing actions into a custom disaster and how events function. Here is another example of some actions in an event:
# This is event 1 which will be run as soon as the disaster begins, all of these actions will be run with event 1event 1:# This is a disaster action this will start a level 5 acid storm silentlydisaster 1:type:'acidstorm'level:5broadcastAllowed:false# This is a second disaster action so this will also start 4 level 6 earthquakes on random playersdisaster 2:type:'earthquake'level:6target:'random'targetAmount:4offset:1# This is a broadcast action, this will send everyone in the world the following messagebroadcast 1:message:'&cYou feel a cold chill down your spine..'target:'all'# This is a sound action, this will play a sound. More about this in the Sound sectionsound 1:type:'block_anvil_land'target:'all'volume:0.5pitch:0.5# Amount of times the sound will repeatrepeat:5# Delay in ticks between repeats (20 ticks = 1 second)intervalTicks:60location:x:~5y:~0z:~5