What is Ruby in Heroes of Mavia?

Could you please clarify for me what exactly Ruby is in the context of Heroes of Mavia? Is it a type of in-game currency or resource? If so, how is it acquired and what can it be used for? Additionally, are there any specific benefits or advantages to having Ruby in the game? Understanding the role and significance of Ruby in Heroes of Mavia would greatly help me in navigating the game and making informed decisions.