HB got 2 things in this game.
1. You use it to lv up heroes to next star
1-20 1Star (upgrade cost 1000HB)
20-40 2Star (upgrade cost 3000HB)
40-60 3 Star (upgrade cost 10,000HB )
60-80 4 Star ( upgrade cost 20,000Hb )
ETC
When you reach max lv of your star you need pay Gold, HB to upgrade your hero to get next star lv . If you Got lv 20 hero and you dont upgrade it to Next star your her will always be lv 20.
2. You can use HB to Roll Heroes in Hero Altar but this is wasting your HB