It's free to play, you download it, create an account and play it. You are given the first "hero" for free, and unlock more through gameplay. The heroes are divided in rarities, as typical in mobile/gacha games, classified in star levels. Five-star heroes seem to be the rarest and therefor, coolest/sexiest/strongest, and almost impossible to get without investing a healthy amount of real money into the gacha system, which I suppose it's where they'll get their revenue from, along the xp boosts necessary to bring both the heroes and their weapons up to the max level, which you can also get slowly through gameplay, or just pay for the instant boost.
It's a game prone to whaling, that you can play and enjoy at your own pace, knowing that others will pay more than you and get more, better and easier rewards because of it. If that doesn't bother you, you may still enjoy it. As I said.. not my thing, but again, it's getting great praise from regular users, so there may be something to it.