"You can lead a horse to water, but you can't make him drink" means that when someone is offered an opportunity, there is no way to force the person to accept it. Often, just the first seven words are used.
According to Phrase Finder, this saying is the oldest known English proverb that is still said in modern times. The first recorded use of the phrase was in 1175 when it was written in Old English as "Hwa is thet mei thet hors wettrien the him self nule drinken," which translates to "Who can give water to the horse that will not drink of its own accord?"
The phrase has been used commonly in language and literature since the 12th century and was included in the 1602 play "Narcissus."