Recording into Audition from iMac High Sierra

If all you have is your Mac's built-in mic and you don't want to use it then, yes, you will need an additional piece of hardware; an audio interface (and a mic and/or other audio source).

Depending on how recent your Mac is, there are easily a handful or more of interfaces that will connect to it via a Firewire, USB or Thunderbolt port. Here's a place to get started: https://ehomerecordingstudio.com/best-audio-interfaces/

Also, depending on what type of recording (and production, if any) you want to do, you'll also need software (known as DAW, for Digital Audio Workstation). I believe Macs come with Garage Band, and that may be enough for your needs. But there are more than a handful of others.

One thing I must stress (as the article does) is COMPATIBILITY: while most interfaces are compatible with most DAWs, you also need to make sure BOTH are compatible with your computer AND its current version of the operating system. Nothing's worse than spending a few hundred bucks on hardware and/or software only to discover they don't work with your computer. Compatibility can be difficult and frustrating, but it's important. As time goes by, and you decide to update anything, you'll have to research the compatibility again. Just keep that in mind. ;-)

Let us know how it goes. Have fun!