Bouncing Colored Beach Ball Appearance ?

[Robert11]

Hello,

If I asked this before, I apologize.

Have a few years old mac desktop
Works fine, but-

Seemingly at random, I get that colored bouncing beach ball.
Hard to correlate its appearance with anything in particular I may, or may not, be doing.

Drives me crazy.
"Locks up" the mac.

What is it ?
What causes it to appear ?

**What can I do to stop its, random, appearance ?

Thanks and regards,
Bob