Currency is a trading item that can be used to purchase items with. Some games have currency implemented so that players can buy or sell items.
In RPG Maker, there is a default system for currency implemented, as well as a shop mechanic and the ability to change the amount of currency the player has using event commands. In addition, enemies can be set to give out this currency when defeated. Variables can be used to implement secondary currencies when combined with event commands or scripts or plugins.
The default currency in RPG Maker is titled "G", and this can usually be changed to a different term in game using the database or scripts or plugins, to for example explicitly title it gold, or rename it to crystals, US dollars, or the universe's actual main currency, so that it matches with the game's setting.
There is a built in limit to the amount of currency that a player can have in game.