The goat pens is a camp building that earns you 2 2 milkresource every hour from when you last collected it. This includes when you are offline, so if you collected the milk at 7:23, you could collect more at 8:23, whether or not you were online during that time.
This structure also shares a unique trait with the chicken coop where it spawns an animal companion in your camp; in this case a goat. This is purely cosmetic and offers no benefit itself.
Trivia
During beta for a couple weeks after goats were released, there was a bug where goats remained even after the owner left the camp. Also during this time, goats would approach players, even if it meant leaving the player's area. As such, it was not uncommon for over 20 goats to be present; all clustered around a player that had been there for awhile. This caused a lot of lag for some players, although many were also a bit sad to see the bug fixed as it was quite adorable.
2 milk resource every hour from when you last collected it. This includes when you are offline, so if you collected the milk at 7:23, you could collect more at 8:23, whether or not you were online during that time.
