In Japan, a Medal game (メダルゲーム) is a special type of arcade game in which relies on the concept of "medals" (tokens, metal coins). Users exchange money for medals, which can then be used in games that give out prizes - more medals, which in turn can be spent on other games.
The idea is to strike a balance between a traditional arcade game and gambling with real money. As medals have no cash value and cannot be used outside of an arcade, it allows operators to circumvent gambling restrictions, while rewarding player skill (or luck).
Sega has been producing medal game machines since 1986, when the concept began to see widespread adoption across Japanese arcades. They are significantly rarer in other parts of the world.