It means¹ you can give someone the opportunity to learn or to do something, but you can never force him to accept that opportunity.

Eg: Greg always complains about not having enough money. I showed him an ad I saw in the newspaper for a part-time job. He said it was a good idea, but he never called the phone number. I guess you can lead a horse to water, but you can’t make it drink.

Note:

这句话的原意是你可以将马领到水前，但你无法强迫他喝水，后表示你可以给一个人学习或者做什么事情的机会，但是你无法强迫他把握住这个机会。在中国有牛不喝水莫强求的俗语，意思相近。

例：Greg总是抱怨没钱。我给他看一张报纸上的兼职广告，他说不错，可从没打过电话咨询。真是牛不喝水莫强求。