Could someone walk me through every thing on the back of my amplifier?

Someone gave me her old amplifier (Kenwood KR-A4050), so now I’m looking into getting some speakers and a subwoofer. I have around zero knowledge of audio systems.

Preferably, I’d like to hook up four speakers and a subwoofer. However, browsing around the internet I’m starting to be afraid that I won’t be able to connect the latter. I’d like to understand more of what everything on the back of the amplifier does, so I was hoping someone could walk me through it.

The back looks like this.

So I believe the A and B rows are for connecting different sets of two speakers? And to connect a subwoofer, I’d need a line out/pre out or something? Please enlighten me.