A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube (or valve in British English) with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device, which propelled the electronics age forward, by enabling amplified radio technology and long-distance telephony.Triode_vacuum_tube.png
Filament Heats up Cathode - Negative Charge
Plate or Anode is positively charged.

Electrons flow quickly to Anode increasing overall voltage... no resistance in vacuum tube

Put a Grid in-between
The Grid is sent positive and negative current based on waveform
When in positive, the electrons speed up to the Anode
When in Negative the electrons slow
This change in flow or (bias) is amplified in the signal coming out the anode and to the speaker.

Small current of audio signal enters a vacuum where a higher load of current is flowing.
The polarity of the audio signal controls the flow of the current inside the tube thus replicating the signal at a higher voltage.

How it works!
Part 1
Part 2
Part 3