From Quake Wiki
Their most basic function is to simply be triggered by another entity, then trigger another. This on its own is a useless behaviour since you can have the two original entities simply trigger each other, however, some additional controls from the trigger system give you more control over what happens.
They duplicate all the behaviour of other triggers, but do not use most of it since they are a non-solid point entity. This is their only real reason to exist - saving you from including another trigger brush in your map, since you can flag a trigger_whatever as non-solid.
targetname - What is this relay called, so that another entity can trigger it
target - Will trigger this entity when triggered
killtarget - Will remove this entity when triggered
wait - wait this amount of time in seconds before triggering the target
message - print this in the centre of the screen when triggered
Bug: No Killtarget + Target
If you give a relay both of these values then the killtarget will not work. You need a qc fix or to use two trigger_relay's - one that kills and one that triggers.