HGG Community Forums

I think it is related to the amount of living horses produced by your barn, but that doesn't always match up.

You get exp every time you create a horse. I think roughly 1 exp per horse. Buying pre made does not add this amount. Selling horses to the auction will detract from your total exp.

Once you gain enough exp you get an additional stall added to your primary barn.

I've been playing for about a year and a half, so I know what Sandy is talking about.
The counter used to start at 200, but it would reset at 100, so it has always taken 100 points to get each new stall into your primary barn (I'm at around 22, and it still is, so don't worry, the limit won't rise as you get more).
For every horse or foal you create or breed, another point is added, and for every horse deleted from the game by directly selling it or by auctioning without any bidders, a point is removed.
You can lose a stall if you get up to a level long enough for it to be added and then delete enough horses to get below it again. :)

Experience points do nothing else, and nobody can see yours unless they look for it, so don't worry about it. It's just a nice way of rewarding players who create or breed a lot and don't have a big turnover. :)