Stained horse glasses boosts a certain trait of the foal that's about to be born (int, scp, spd etc) You are to match weaknesses of one horse to another horses strengths in the hopes of getting a WWW but random factor plays a part in it so it doesn't always happen
When I was looking in people's barns for nice broodmares to breed to some of my stallions, I came across a few mares that showed the 'Heart' public breeding icon but when I went to see the breeding price of the mare it showed she wasn't for breeding. She had the icon though so this makes me believe it is a bug.