is the an update of the operating system of just an app update?

Tucsonmac wrote:

to my imac running high sierra

In that case you need to update the iTunes application on your Mac.

Connect the iPod to the Mac and it should tell you there is an update required. Just accept the update. It will update iTunes.

You don't need to update the operating system.

Connect your iPod to what exactly?

If planning on connecting it to a computer, you need a recent version of iTunes.