You asked what relay means so ima try to explain it
Basically, when you do the output event fireRelay it simply triggers onRelay. Thats all.
So
0 | onActivate - Self - fireRelay
1 | onRelay - Self - fireRelay
2 | onRelay - Self - spawnProjectile [rocket, 0 0 100, 0 0 0]
will keep shooting rockets in the sky after you clicked it.
You can also fire relay in certain directions (North, South, East, West...). That will trigger the event onRelay on a brick that is next to the first brick in a certain direction.
You can also do this event
Brick1
0 | onActivate - NamedBrick [Brick2] - fireRelay
Brick2
0 | onRelay - Self - setColor
That will turn Brick2 red after clicking Brick1.