What's going on when my mac's rainbow disc keeps spinning?

Basically, from what I understand it’s when the computer’s internal calculator isn’t able to calculate when a process will be done.

Mostly a clog of digital traffic, or your RAM has been maxed out and it’s reassigning memory.

Or, and application has completely crashed. Rare though.

Most common problem is that the application is hanging. This happens in Firefox more often than it should.

It’s affectionately called “The spinning beach ball of death”.

The application is not accepting new keyboard/mouse/system events from the operating system. Applications run in an event loop, where they accept an event from the OS—a keyboard push, a mouse click, a system notification, or what have you, and then process it. Sometimes this processing takes longer than it should, and the application doesn’t get back around accepting events for a few tenths of a second.

If you pay attention, the beach ball only appears when you have the mouse over one application. (Unless something in the system’s event handling gets screwed up, and the same problem causes multiple applications to get wedged and to not accept events.)

or the pinwheel of death.