Well, it goes like this. The amplifier or preamplifier modulates the power supply. That doesn't mean much now. But one of the first things you put together in just about every Bottlehead kit is the power supply. It consists of taking the output of one of the transformer windings and running it through either two or four diodes. That makes really lumpy DC.
Aside AC is alternating current, DC is direct current. The definition of alternating is just that it is both positive and negative during a cycle. Direct current is either only positive or only negative.
But the output is really, REALLY lumpy. It needs smoothing. The following capacitors and resistor(s) smooth the DC. It looks more like a steady state voltage (no lumps).
The tubes are single ended triodes or pentodes. A triode has three leads (tri- part), a pentode has 5 leads (pent- part). Single ended I'm not going to define because I always botch that up. Anyway, there is a steady current through the tube from the plate (top?) to the cathode. This is caused by the DC across the tube and controlled by the resistor between the cathode and the ground reference, where the negative, or common of the power supply is soldered to.
The input goes to the grid (we now have mentioned the three parts of a triode, plate, grid & cathode) and the voltage on the grid controls the current through the tube.
When you get your kit post back and tell us what kit it is. If I have the schematic I can point out these elements to you as you build.
Looking at my Eros manual I see that the sections are named Power Supply PC Board, Plate Load Servo PC Board, Chassis Wiring ... So it does give some idea of what you are working on. I hope your kit does that as well.