what is nss?

i see it in multiple clubs and such but what is nss, where do i find it and how does it work?

nss stands for non-specialty stats. If a horse has a specialty set, say Driving, they're specialty stats would be STR and END. The amount of other stats, SPD, AGI, and INT, combined are the non specialty stats. They aren't really important, and it doesn't really matter how big or small they are, unless you're showing your horses. I won't explain how nss matter in shows, but I'll link to this, which should explain it far better than I ever could. 
I don't know if this makes sense or not, but hopefully it gave you an idea of what they are.