There are multiple server traditions used on the server. Some, however, are falling apart.
Hitting Bedrock Edit
When a player hits bedrock in a mine, everyone else on says "Congrats".
Finding Diamonds Edit
When a player finds diamonds in a mine, everyone else on says "Congrats".
IDH stands for "I don't have". It's a quick way to state that you don't have an item someone else wants.
Say "Bed" when you are in your bed, alerting others that you are in your bed and others should do the same