A hero call is an act of calling a bet on the final betting round, knowing that you have a relatively weak hand but suspecting that your opponent is bluffing.
It’s called this because you’re making a brave decision and putting all of your faith into a very uncertain situation.
For example, you’re holding a suited 78 hand, and the board comes to a 10, 7, 2, 9, and an A. Your opponent fires a bet on the final betting round, and you decide to call suspecting that he’s missed his inside straight draw.
After the betting round is over, your opponent turns over their cards to unveil a QK, allowing you to win the pot thanks to your brave hero call.