The /helpop command is available to all players. It sends a highly visible message to all online admins from a player in the event of an emergency.
How to Use
Press T and type /helpop <your message here> then enter. For example:
/helpop A player is threatening me now by writing bad things on a sign!
When to Use
When an emergency is happening. Some example of emergencies are when you are being harassed (spammed teleport requests by one person, someone is at your unprotected home at that moment griefing it, a player won't leave your home after being asked, you are 'stuck' in one spot, or you are being threatened or made to feel uncomfortable in any way), or there is a threat to the server's stability.
When Not to Use
If you only want to talk or ask an Admin a question that isn't an emergency, or need anything usually done by submitting a modreq. This should not be used to chat with a vanished admin outside of an emergency.