ChatterBank0 min ago
No best answer has yet been selected by michysj. Once a best answer has been selected, it will be shown here.For more on marking an answer as the "Best Answer", please visit our FAQ.
The phrase in its literal sense means that you have control over where the horse will or will not go, you have reins and can lead it to water. Once there, the choice of wether or not to drink the water is down to the horse alone - you have no control over that choice. It's used to illustrate an argument that you may have a degree of control over a situation, but somewhere along the line, there will be an element of free will over which you have no influence.