What Does This Error Mean?

Does the screen look something like the following?

If so, that would indicate that your Mac has experienced a kernel panic.

To get started, check out the following article to see if it can help:

• If your Mac spontaneously restarts or displays a message that it restarted or shut down because of a problem - Apple Support

Typically, but not always, kernel panics are caused by missing or corrupted device drivers. One simple test, is to remove any peripherals from your iMac except the keyboard and mouse to see if the panics stop. If they do, then add them back one-at-a-time, until the happen again. That would identify the culprit.

Look in /Library/Logs/DiagnosticReports/ for a file whose name starts with Kernel and has a .panic extension.

Open the log and copy the text. Paste it into the Additional Text button in the toolbar of a reply here.

You're welcome but if you want to dig down to why your Mac is presenting this screen, then Barney-15E's excellent suggestion would be the next step to troubleshoot it.