This article is about the kind of cable you need to plug your iPhone or iPod or laptop into a stereo.
If you look on any iPod, or the back of a Mac mini, or the side of your Apple laptop you will see a round hole where you can plug in headphones. iPhones unto the iPhone 6 have a headphone socket as well. For later iPhones (the iPhone 7 and 8) you just need this adapter which turns your lightning connector into a headphone socket, and then the rest of this article applies to you as well. If you get a dock for any model iPhone including 7 and 8 it will also have a socket like this on the back.
The technical name for this socket is a 3.5mm or 1/8 inch stereo jack.
You can see one of these on the left of this iPod dock below: