Beats Me!

What do you know, as a writer, about "beats"?
And why do so few people seem to be able to use these things?

Second to the myriad errata I see in manuscripts of late in terms of commas, word usages, homophone confusion, and action/reaction transposition--and there are tons of THOSE--comes an embarrassing number of writers who, apparently, have never been taught about beats.

(Some of them haven't been taught about dialogue tags, either, which are related...but not quite the same thing.)

Do you know what a "beat" is?
Can you use it effectively?
How would you describe it?
And how do you learn how to write "beats" well in your prose?

I've got thoughts on this (yeah, like you ever doubted I did?), but I'd like to hear from other writers on this. This problem is so prevalent that I'm beginning to think NO one's taught it for at least a generation of writers...maybe more.
If so, I know what I'm doing my next workshop on.

Chime in if you would!
Janny