Re-amping is the process of taking an audio signal out of the computer, run it through guitar pedals, amps, effects, etc and then record it back into the session. This process is more common on guitars, to do so all you need to do is record a DI.
Why would anyone re-amp guitars?
Well, lets say when the recording was done, the artist didn't successfully capture the tones they were looking for. Then a mixer with a better amp can do a re-amp and achieve that sound the guitar player was looking for, that makes a record sound great.
Some producers like Joe Baressi like to re-amp drums through guitar distortion pedals for example. Other artists like The Mars Volta would send vocals, drums, synths etc to pedalboards and then back to the computer. Listen to the sound design on "Deloused in the Comatorium", they have so many textures and great trippy sounds that came from them messing with guitar pedals and re-amping sounds. Another famous engineering trick is re-amping a layer of distorted bass and layering it under your bass tone.
This process of re-amping guitars is very simple. It starts with sending your DI (this is the recording of your guitar straight from the guitar, not going through any amp) out of the computer and changing its level to match your destination. If it is coming out of the interface (line level) it has to be transformed to instrument level (same level that would come out of your guitar). I use Radial X-Amp. This box lets me send a balanced signal, converts it to a guitar level that is unbalanced and I can plug it straight to my amp. It even has a little boost knob where I can push an amp from.
From there I can go to any pedal or amp. In this case Im going to use a KHDK Ghoul Jr, into a Marshal JCM200. Using a long speaker cable, i go into my ISO Box.
Inside my ISO box I have a Mesa Cab that has two microphones. Then they go to my Capi VP26 preamps and I record both channels into the computer.
This process lets me dial in a guitar tone that fits the mix Im working on better than what the band had previously recorded.
I love reamping sounds. I like to do Vocals through my Hairball Audio 1176, Bass through my SansAmp and a Big Muff, etc. The idea is to get creative and explore new sounds with every mix.