Cancel/Uncancel event is an idea of command that is supposed to cancel the reason of execution of a command prompt. For example, used in a "get hurt" command prompt, this command will make the game act like if the enemy never get hurted, however, the commands still be executed.
This is for example used in this:
When you damage an enemy
sound = upload 1
<enemy 1>Ouch!
cancel event
The sound is executed, it says ouch but the damage is simply not received.
Uncancel event is the same command but it's used when the event has been canceled, this nulls any cancel event before, anyways, it can be canceled again.
Source where I extracted this idea:
https://skriptlang.github.io/Skript/effects.html#EffCancelEvent