To upgrade your Heroes and purchase some chest packages (which gives you Hero Souls) in the Shop. In essence, you need to pay two forms of currency to upgrade your Heroes. First, you need to collect sufficient souls, then you can pay gold to convert those souls into a Hero of a higher level.
Where do you get it?
You can receive it periodically from the Gold Mine.
Boost is the feature which uses only Soul Dust as a resource and gives a short term (24 hours) advantage to any hero in your deck by levelling the level of the hero by 1. The cost for the next use though raises substantially each time and resets when the effects of the previous use(s) wears off. For more information about Boost, click here.
Soulbind, on the other hand, is a feature which also uses only Soul Dust as a resource but gives a permanent advantage to any hero in your deck. You can do Soulbind 4 times for each hero and the cost of Soul Dust increases respectively depending on both rarity and the level of Soulbind. For more information about Soulbind, click here.