I see lots of talk about it with mosins. What is it,  how do you tell it was done, why was it done and why is it considered bad thing?

Counter boring is the "reaming" of the muzzle end of the barrel. This removes the bad crown and worn rifling in the end of the barrel. It's easy to see as the rifling will be gone for 1/2 to 1 inch inside the muzzle.
This can be beneficial from a shooting stand point as it has a "new" crown albeit recessed into the bore. Most M38's and 44's have been counter bored (that I've seen) but few 91/30's.
Some collectors say counter boring reduces it's value somewhat. Some collectors don't care.

Unless you are a serious collector or like to sell/trade guns often (I don't, I keep every one I buy),  as long as the rifle is accurate IMHO, doesn't make a difference if it is counter bored or not. My 91/30 is not, my m44 is, I can ring 12" steel plates at 500 yards with both with no issues

One of my Mosins is counterbored about an inch down the bore.  Still shoots like crap.  Counterboring can improve accuracy, but it didn't in this case.

If a rifle needs counterboring, but isn't, then you have something authentic to hang on the wall, but not shoot.  If it is, it has a better chance of being a decent shooter.  They aren't all good shooters, anyway, but sometimes that is because there is too much clearance around the barrel, and it needs shimming or wrapping.
None of mine are counterbored, but it's not really that much of a stigma.  After all, these rifles are 70+ years old, and some of them have been through hell.

SK

CB = a problem with the rifle was corrected.

I have many a mosin nagant that is CB.. as a collector it doesn't bother me.