A Little History and Theory
The triode descends from the tube diode (tube rectifier) and as a consequence it shares several features with the diode. For example, it can only conduct current in one direction: from its cathode to its plate (or from cathode to grid when the grid is more positive than the cathode). In addition, it offers some resistance to the flow of that current, which means that like the tube diode, the more current flowing through it, the greater the voltage drop across it. (In fact, if we tie the grid to either the cathode or the plate or if we leave it unconnected, the triode is reduced to a diode.) This resistance is referred to as the triode's rp or plate resistance. (In the old days, rp was reserved for references to the triode's AC impedance; and Rp, the triode's DC resistance. Sadly, today, rp covers both the DC and AC aspects of a triode's behavior.)
By allowing the easy control of the current flowing through the triode, the grid makes the triode a useful electronic device. The poor relation, the diode, lacks this feature, as the only way to control the flow current through the vacuum diode--other than to reverse the polarity of the applied voltage--is to vary the voltage between its plate and cathode. In fact, if the diode did not hold the property of unidirectional current conduction, it would be of no more use than a slow-turn resistor, with a relatively short life expectancy and poor linearity. In contrast, by simply varying the voltage on its grid, the triode's current can be completely stopped -- or increased up to complete saturation.