Exactly what is a Pre Out?

My brother bought an integrated amp, an older one, and wants to install a set of Pre outs for a subwoofer. How does one go about doing this? Thanks in advance.
If preamp outputs aren't already present on the integrated, it is probably not a good idea to try to add them. You'd need to solder a new connection at just the right point on your amp's circuit board. That's not a job for your first time with a hot soldering iron inside an amp.

Unlike line out (or tape out) which has a constant signal strength, the preamp out would track with the volume control. This would control the volume of your sub in sync with your main speakers as you change volume.

Many subs have a connection that runs off the speaker wire terminals. This would be your better option if the integrated isn't already configured with pre-outs.