A Hero is a mortal who has been imbued with a little divine power by a god or similar divine powers from the Maelstrom. In the times when gods visited the mortal realms, some had relationships with mortals and their children were Heroes. Hercules was one of these, with Zeus as his father.
Not all Heroes have divine blood in their veins though. Many are normal mortals who are given a touch of divine power by an immortal patron, whether they know it or not.