It is a valve that opens and closes in response to the input signal fed to it. Output side, (anode or plate), is connected to a high voltage power supply, that feeds the output transformer connected to your speaker through the tube. The input side, (grid), accepts the tiny input signal from your preamplifier, and turns the large power supply on and off in response to that input. The result is a much larger signal being applied to your speaker , than the tiny input signal being applied to the grid. Basically how amplification of the tiny input signal is achieved.
Of course there are more parts to the tube that sets the amount of current bias is applied. But the above is a simple as I can attempt to explain it.