I1) You don't have to indicate "Every Tick". if conditions are met the event works, since on the 12th line there is no condition it will always work = every tick
2) https://www.scirra.com/manual/126/system-expressions , as you can see find function returns -1 if it cannot find anything.
Just click on Player from you Objects tab and change any value you want, it will be the default value.
So how does it work? You seach if UID of the enemy is already written in your hitid variable(which is freshly created each time you swing your sword)
As I told before the default is , because I'm always checking for ","&UID&","commas are important otherwise you can't differentiate between single, double or triple digits. That's why after a collision you add UID&"," instead of just UID
, number1, number2, number3, number4,......(without space) as you can see now each number is enveloped.
ps: UID is the unique id of an object, it's different(well unique :p) for each object so it is useful here.