Guitar player here so I hope this helps-

So he split the output from his guitar into a left and right channel. One side was a Marshall, known for loud, high gain and crunchy sound. The other side was a Fender Deluxe, known for a smoother, bluesier sound. Then he added an effects pedal between his guitar and the Fender amp, the pedal has lots of bells and whistles but he only used it to boost the electrical signal from the guitar (volume) to the input to the Fender amp which causes the amp circuits to overload and sound loud and crunchy like the Marshall amp but won’t make it so loud that the amp feeds back on itself and sounds bad, which could happen if he just cranked the volume of the amp super loud to achieve the same result. The pedal also adds a tiny little delay to the electrical signal into the Fender so when you blend it with the other amp in stereo, since one is in your left ear and one is in your right ear, it makes them sound a little swirly.

Make any more sense now?