Take the Mares current level and the Stallions current level and add them together.   Then take the resulting number and divide it by 10.   If your number has a decimal round it down to the nearest whole number.
(Mare Lvl + Stallion Lvl) / 10 = TB (Training Boost)
Example:

(50+75) = 125 / 10 = 12.5  (round down)   Foal has potential for a max of 12 TB.  Foal must fully complete foal training to reach this max potential.  If foal is 6E (Excellent BS) the foal will also gain +2 TB

We have a PS formula for breeding that gets us close to  proper results, but it is not a 100% perfect result.  Instead this lets you see a close number to what they will get, but expect some wiggle room.  This formula is better  known in the Discord Server.

(Mares PS + Stallions PS) /4 = Foals PS (Roughly)
Example:
Mares Endurance + Stallions Endurance /4
119+20 = 139 / 4 = 34.75 = Foals endurance will be in the 34-35 area range

There are 2 different formulas on this page, I will leave this one up to if you want to learn it, a lot of players don't worry about this portion.

Parents are groomed
((Mare BS Value + Stallion BS Value) / 2) +/- 4 = Foals potential BS Range
Parents are ungroomed
((Mare BS Value + Stallion BS Value) / 2) +/- 6 = Foals potential BS Range

The above hyperlink also contains extra info about BS Ranges and how to find BS / Ideal BS

If doing this yourself doesn't strike your fancy, using Walrus' BS Range Calculator is a solid option too.

Example:

((Mares Leg Value + Stallions Leg Value) /2) +/4 = Foals possible Leg BS Range 

Breed Variance uses your horses personal BS numbers and compares it to the breeds Ideal BS numbers.   Normally breed Ideals need to be found on your own, but there are plenty of players who have this information shared.

Once you know your horses BS and the Ideal Breed BS that matches your horses Breed you find the difference between your horses body sections and the ideal body sections and then add those number up.   This is visually shown through the star system.