[crackle]

"that capacitor" is a meaningless and confusing term used by many on this forum. You don't see it mentioned in any of the original service manuals.
I hate the term, and like you, when I first came across it I had no idea what people were talking about, there are lots of capacitors in a radio, which that capacitor do they mean.

The term could refer to any capacitor that happens to be the subject of a conversation. But often the writer is referring to the "audio coupling capacitor" which connects from the anode of the preceding valve (often the audio preamp) to the grid of the audio output valve.
The purpose of the audio coupling capacitor is to allow the audio signal (AC) to pass through it, but to block the high voltage DC found on the anode from getting to the grid of the output valve.
The output valve normally has a negative DC voltage on its grid, to "bias" the valve, and any positive voltage here can cause failure of any of the following; the valve, the output transformer, speaker energising coil, smoothing choke, and possibly the mains transformer.

So it is essential to change the "audio coupling capacitor" to one that has zero DC leakage so it can continue to block the positive voltages getting to the output valves grid.

Some older radios may use an audio coupling transformer instead of a capacitor.

Mike