Merge mayor has 2 currencies. Gems (hard currency) and coins (soft currency).
While both can be used to buy items from the shop or to unlock storage slots, gems can be used as an alternative when ad options aren't available (to unbubble items, upgrade daily rewards, recharge players' energy to 100) or for skipping generators' cooldowns. Coins on the other hand can be used to upgrade one's production buildings.
If players wish to increase their in-game currency, they can either buy them from the shop or obtain them from their daily rewards or level-up rewards.