"Coupling" refers to which signals can or cannot be passed through a given patchpoint. "AC coupling" means that the point can be used for anything in the audio range, which can vary from manufacturer to manufacturer, but in general it implies a cutoff that blocks DC signals (or anything in the range below subsonics, such as modulation). "DC coupling", however, means that the patchpoint can accept any signal, all the way down to DC (offset voltages, y'know).
And this can get interesting when you've got a bunch of DC-coupled devices in your voicing, but NO AC-coupled ones after that to strip DC, then you send it on out to your amp. And this can result in............
Yeah. Always try to eliminate any and all DC in your audio path, because once it hits your amp, things WILL get ugly.