I think @gapplegal summed it up. Cubits are a global currency, and are basically ‘store credits’ - Super handy if you want to purchase an item or rank on the store, but aren’t able to with IRL money.
They can be hard to get, either through voting, ads (which are unreliable), and trading with players in-game, as @JustSoDank also mentioned. A whole cubit (1.00) is equal to an IRL US dollar, and each 0.01 cubit, is equal to one cent. Even if USD isn’t your currency, cubits are technically measured in dollars, so when paying with cubits, it’s probably best to use USD to ensure you have enough!