A Port. The Amp includes, well, an amplifier so that you can use it to drive a non Sonos set of speakers. 

But frankly, you could get the turntable’s input by plugging the CD deck into your Five’s line in, too, without buying the Port. 

And further, you’d actually be better off ripping your CDs to your choice of formats, storing them on either your computer’s hard drive, or an NAS, and just play them digitally, without needing to connect the CD deck up to your Sonos at all.