It means that she is giving birth to a foal, or baby horse.
If you mean how long are mares in gestation, then 11 months. If you mean how long until a mare can breed again after foaling, I don't really know. The mare goes back into heat usually around two or three weeks after foaling.
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foaling
Foaling.
The mare should come back into heat (not including foaling heat) 21 days post foaling.
Post foaling, the mare's uterus needs to discharge remaining fluid and return to it's original size (involute). By around 10 days post foaling the mare has gone through foal heat and completed the "cleaning stage".
To calculate a mare's estimated foaling date, you can use the mare's last breeding date as a reference point. A mare typically has a gestation period of around 11 months (340 days), so you can add this to the last breeding date to estimate the foaling date. It's also a good practice to have a veterinarian confirm the pregnancy and monitor the mare's progress.
Preparing your mare for foaling in the proper way is essencial in her health. I have written an article on when my mare foaled, and how we prepared & cared for her, and our new foal. This is the link: http://www.ehow.com/how_4526561_prepare-mare-foaling.html
Ideally, foaling takes place in a specially built large stall called a foaling stall. When foaling outside in a herd, mares often separate themselves to foal. A few seek out the company of a special horse friend (usually another mare) or the herd while foaling.
To give birth to a foal (baby horse) by a mare (mother horse).
On Howrse Quiz it's Straw
There are several foaling calculators available...one on The Horse.com