The signal enters the grid and leaves via the cathode-not the plate. But capacitors don't let all AC through. Capacitors that are of a small value let just high frequencies pass and block lower frequencies. Larger value capacitors, or caps, let more of the lower frequencies pass along with the highs. Think of capacitors as a kind of window or door-the larger the opening or value, the lower the frequencies allowed through.
Capacitance is measured in Farads.
Most capacitors have micro Farad ratings. Thus, capacitors are very useful in tweaking the tonal response of circuits. This is one of the easiest modifications you can do. Just make sure you're dealing with preamp circuits, not power amp circuits. If wanted a more trebly sound, simply decrease the value of one or more of the coupling caps.