No worries. There's a kind of alchemy involved. I don't understand it all either but with gain you can transform the nature of the flow of electricity. Turn up the signal strength (gain) and you open up the overtones and the sound becomes more complex.
The silver pedal on the right is the one that is used for volume control or gain if you like. The Wah can be used in a similar way but it will transform the tone and not the volume so much. All sorts of desirable effects will come from manipulating the signal characteristics.
You might find a fuzz pedal does a fair job and you can use the guitar's volume pots in the same way as a volume pedal is used. Go in to a shop and try out the fuzz pedals. Try and have the sales person plug in to an amp just like the one you want to use. See how that goes.
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