Voltage level in Britain's ERNIE
The grid with the red dot is similar to the gate in a JFET; the anode is like the drain, and the cathode is like the source. The other grids are there to optimise performance and would not appear in the operational equivalent circuit. Quiescent DC through the valve (and hence through the 1k) causes the cathode to be +ve wrt ground by a few volts, and this makes the grid -ve wrt the cathode (almost zero grid current flows through the 470k). This is the grid bias voltage. The quiescent voltage measured at the grid, relative to ground, will be zero.

Basically, the heater warms the cathode, which creates an electron cloud; the anode attracts the electrons; and the grid controls the flow of electrons from cathode to anode. Auxiliary grids are there to accelerate the electrons and to confine the electrons to the anode's neighbourhood once they've zipped thorough the grids.