The breeder's advice is a piece of text found under the Genetics tab on your horse's profile on Horse Reality.
The text is written automatically by the game depending on the horse's genetic potential and conformation stats. For example, a "very good" walk stat triggers the sentence "[Horse name] has a powerful and balanced step" while a "below average" walk stat triggers a different sentence - "[Horse Name]'s toes are dragging just a touch". Each of the 10 genetic potential stats and almost all of the 12 conformation stats trigger a specific sentence based on their value. As a result, the game calculates a recommended discipline for the horse to be trained in and compete.
The G+ category is player-made but very popular. There are no specific sentences for "good" stats themselves, instead, they trigger either the sentences for "very good" or "average". Depending on that, players often differentiate between the following:
Note: Neck and shoulders are two separate conformation stats, but in the Breeder's Advice they share a sentence.
The game calculates which discipline(s) a horse should do best in and lists them at the bottom of the Breeder’s Advice. This is only a recommendation. The actual performance will also depend on factors like the strength of competitors for that discipline on your continent. Thus, the breeder’s advice is only a loose recommendation - asking experienced players in the forum for a discipline recommendation is usually the better way to go!