Its the space between the pixels in the screen of the same color. A monitor with a .21mm dot pitch means there is .21mm between one red pixel to the next. The are different ways to measure dot pitch also, such as horizontally, vertically, or diagonally. Pixels can be arranged in a number of ways.
It's not huge, but it's probably noticeable at a high enough resolution.
I'd recommend ViewSonic or NEC -- they both make excellent CRT monitors. I use Viewsonic at home, and my office is all NECs, and they're both pretty good (I give slight preference to Viewsonic because they tend to be a bit cheaper). I haven't used a Samsung monitor in a *while* (other than for maybe 5 minutes at a time in a computer lab somewhere), so I can't comment there.
Lower is better in regards to dot pitch on a monitor. The closer the dots are together (the smaller they are) the more detail an image can contain. As well as the monitor gets bigger (21") the dot pitch generally is less than say at 17".
Take a look at a TV up close. It is all fuzzy and blurry and you can actually see the individual pixels. Wouldn't want that on a monitor would ya.
That being said I have a 17" Viewsonic PF775 with a dot pitch of .25mm that is @ 1280x1024 and it seems sharp and clear to me.
Do yourself a favor and go read this article...It will give you an overall understanding of what the specs mean...You should take notice of the parts about "aperature grill" and "shadow mask" technologies...