When you have trained a skill as much as you can, in rides or ordinary training, the skill’s label comes up in bold. Hope this helps!

If you do all the rides (forest and mountain), training and competitions that give those particular skills until the horse doesn't gain any more, they'll look bold. The stars are just a visualization of the Genetic Potential

Excellency stars are a measure of your horse's genetic quality, as compared to what the very original horses on Howrse used to look like. Since they're a measure of genetic quality, they're set from the moment your horse is born, and can't change at all. They aren't earnt, completed, or interacted with at all.
They're essentially a visual representation of what level of quality your horse is. They fulfill the same function as the "Genetic Potential" number your horse has.

Maxing out a horse's potential is done by acquiring all possible skills from training, going on rides, and entering competitions, until they stop giving any more skills. Maxing out a skill is called "Bolding" by Howrse players.
Doing daily missions can also give your horse extra skills. These aren't part of Bolding, though.

In your horse's genetic potential info box, and in the Skills info box, it lists the names of all six skills.
Once your horse has gained as much as they can in a particular skill from training, going on rides, and doing competitions that use that skill (but not counting the gains from missions), then the name of that skill will be written in a bold typeface in their skill and genetic potential info boxes; this is where the name "Bolding" comes from. The skill name will look noticeably darker and thicker than the names of the other skills (at least until the other skills are Bolded too).


If you'd like to maximise your horse's quality and help it reach its full potential, then you may also like to help your horse reach its maximum BLUP.
Horses with high BLUP are useful for breeding, because foals of two BLUPed parents are born with higher skills, and can thus achieve higher total skills in life.
To reach the maximum BLUP (which is +100), you have to win at least 20 competitions with your horse, Bold the three skills it's genetically best at, and get it to at least ten years old.
Players have put together numerous schedules for the most efficient ways to BLUP each breed, since getting the 20 wins can be difficult. If you'd like a schedule for a specific breed, you can always ask in the Breeding forum.


Welcome to Howrse, and good luck with your stallion! He's beautiful.

Fire future reference, questions about the game belong in the game play forum. This one is for chatting about real life horsey things.

Some great answers have been given so I am going to lock this now but you can post in game play if you need further help.