Signal path refers to the path that sound makes while being processed.
Recording signal paths include:
Sound Source > Capturing Device > Wire From Capturing Device To Console Channel Input > Channel Volume, EQ, Etc. > Channel Output To Recorder Track
Mixing signal paths include:
Recorder Track To Console Channel Input > Channel Processing, Volume, Pan, Etc. > Channel Output To Stereo Bus > Main Stereo Output Master Fader > Final Mix
In addition, mixing signal paths include:
Channel Output To Audio Bus Through Aux Sends > Aux Channel With The Send Feeding Into It > Processing > Aux Channel Output To Stereo Bus, Etc.
Gain (volume) refers to the way that the volume of a sound will increase and decrease as it goes through the different stages of a recording or mixing path.
Gain structure refers to the input and output levels of each stage of the path. While it’s possible to set gain/volume knobs to anything and at the very last knob turn things down if necessary, you’ll most likely have inefficient gain structure that can cause extra noise to be added in one stage, and even overloading in another.
Unity gain means that a path or even stage of the path has the same volume going out as it did coming in. Although setting a knob or fader at zero will usually provide unity gain, once you start to process a sound you end up changing the volume within that particular stage of the path. You’ll then most likely have to adjust a different stage of the path to compensate.
Many do not understand gain structure and end up overloading early stages, turning the sound down later and not knowing why their meter level is “in the green” but the sound is distorted.
Changing gain means decreasing it (which can be done passively but is usually done using electronics) or increasing it (using electronics to amplify it). Every process a sound goes through will change the tone of the sound, even if the process is a little bit of gain change.