what happens if I don’t update?

You will not get the latest features, security updates, and some apps requires a specific range of iOS version. You may not be able to use an app if the developer of that app no longer supports the old version of iOS you have.

It's not a big deal, but to clarify, iOS is not the operating system for a Mac. That is for iPhones and iPads. Your computer uses a Mac OS. These are completely different operating systems.