CMMG states the following about their stripped barrels:

Barrels have not been drilled for taper pins. linky

What the heck does that mean and what will it mean if I buy a barrel?

Front sight bases are attached on some barrels with a taper pin through the base and barrel.

If you order a bbl with out the standard FSB you will get a bbl that has not been drilled for taper pins.  So, if you want a different gas block with a flip up front sight post there will be no holes for taper pins.

You where looking at their stripped bbls., and they are just that.  If you buy that bbl you have the option on the FSB.

I am thinking of adding rails and putting a flip on site on that... hmmm...

I am building an 18 inch barrel, so options seem limited, unless I want to go above $400.00

the FSB and barrel are drilled at the same time... essentially a drilled FSB and barrel are a matched pair...

the stripped barrels are for whatever gas block you want...

if you want a taper pinned gas block, you will drill it yourself...
if you are using a set screwed block  you will either need to mill the barrel flats yourself (DMR style) or drill dimples into the barrel for the set screws to seat in...

so basically it means you actually have to do some gunsmithing and not just "assembling"

Quoted: I am thinking of adding rails and putting a flip on site on that... hmmm... I am building an 18 inch barrel, so options seem limited, unless I want to go above $400.00

I've used a YHM flip up FSB/Gas block with no problems.  It doesn't require set screws or taper pins, and it has worked great on the bbl I got from Denny.  I put one on a Recon bbl (WOA), and it has never failed yet.