From Quake Wiki
Trigger_counter mainly is used to make a group of entities that targets to another one. You need to target all entities for activate a trigger_counter that links another entity.
Example: You have 3 buttons that activates a door
How to works:
1- To the trigger counter you add "target" and "targetname" values, for example targetname is "cou1" and target is "dr1".
2- Your group of 3 buttons need to activates obligatory the trigger_counter for doing a sequence, so the 3 buttons need have the "target" value that is "cou1".
3- your door is the owner of the targetname called "dr1".
4- You choose in the "spawnflags" if you want message or not(by default: yes. In first spawnflag).
1. No Message: if you want the player does not know the sequence.
count: set the number of entities to make works the trigger_counter (default:2).