Tube amp power supplies are split into two sections: one low-voltage part for the heater filaments, and another high-voltage section for the rest of the tube connections. You should always let the amp warm up with the Standby switch turned OFF. This allows the heater filaments to get the tubes up to proper operating temperature before you apply the high voltage to the plates and grids.

When you turn on the Standby switch, the high-voltage part of the power supply is connected to the tubes. You won't get any sound out of the amp while the Standby switch is OFF, because the tubes don't work unless they have the high voltage connected. That's what the switch is for - letting the tube heaters stay on while the amp is sitting idle (standing by) without applying the high voltage to the rest of the tube connections.

Turning on the Standby switch before the tubes have had enough time to warm up will destroy them.