The directly-heated cathode (or indirectly by means of a filament) produces an electron charge by thermionic emission. This electron stream is attracted to the positively-charged plate (anode), inducing a current. Applying a negative DC voltage to the control grid will repel some of the electron stream back towards the cathode, thus isolating the plate from the cathode; full bias will turn the tube off by blocking all current from the cathode. Conversely, increasing the positive DC voltage on the plate will attract more electrons toward it.
As grid bias is increased, more of the electron current is repelled, resulting in a smaller current at the plate. An AC signal voltage superimposed on the grid will appear as variations in the plate current; voltage amplification can be obtained by using a suitable value of plate load resistance.
The triode is very similar in operation to the n-channel JFET; it is normally on, and progressively switched off as the grid/gate is pulled increasingly negative of the source/cathode.