It's often used to describe someone who:
* Is physically robust and resilient: They can withstand a lot of physical stress and recover quickly from illness or injury.
* Has a lot of stamina and energy: They can work hard for long periods without getting tired.
* Has a good appetite and digests food well: They are generally healthy and have a good metabolism.
The comparison to a horse is apt because horses are known for their strength, endurance, and ability to withstand harsh conditions.
So, if someone says you have the constitution of a horse, they're complimenting you on your overall health and fitness.