eternity.
In Hebrew, the word heart symbolises the mind, intellect and reasoning. And as most people agree, the human experience is one of gaining knowledge and understanding. Therefore, those who choose biblical truth unlock wisdom beyond this reality about the one to come.
He has made everything beautiful in its time. Also He has put eternity in their hearts, except that no one can find out the work that God does from beginning to end.
Ecclesiastes 3:11