what is "unity gain" ?

I think it's when the volume control on your preamp is neither boosting or attenuating the signal.

I beleve you use it for a external processor or amp. It basacally bypasses the pre amp and just passes the signal through so the other componment controls the volume

In simple terms, it's when an "amplification device" (it's usually pre amps) doesn't provide voltage gain, but just sources the current needed into low input impedance loads. This is otherwise known as a buffer. I used the word amplification in quotartion marks as there's no amplification of voltage taking place in such devices. Voltage going in is equal to the voltage going out, thus the unity gain name.

Most such preamps do have a volume control to attenuate the voltage for lower listen levels but they will pass all the voltage from the source such as a CD player, along with more current (as needed) to the amp when the volume pot is fully opened.

I should also mention, that passive pre amps are unity gain devices (since they have no active amplification circuits), but they can't source additional current. This is the reason why many feel that they lack in dynamics albeit beeing transparent. This is also the reason why passives are system dependent in that they must be used ahead of amplifiers that have high input impidances and hence don't draw high amounts of current.

I believe that it is the same as home theater pass thru.