Seriously. I've been using Macs since... ahem... 1985... and did not know this.

It means cancel, stop. You can use it to stop progress bars, dismiss dialog boxes, terminate some well-behaved spinning beach balls, cancel a drag before dropping, etc. It's been on the platform since the classic Mac OS days.

I was wondering why the shortcut for stopping a running program in Xcode was cmd+. Now I know