Behind the scenes
MB Coins are an in-game currency for use within Metal Gear Solid V: The Phantom Pain. The player can acquire MB Coins by purchasing them online with real world money through microtransactions, or instead get them periodically for free via the Daily Logins bonus offered within The Phantom Pain. The DLC depiction of MB Coins depicts them as resembling silver coins with the Diamond Dogs' dog portion of the logo on the front, the year DD was established, as well as the quote "We have no tomorrow, but there is still hope for the future" (referring to the end quote in the Nuclear Disarmament event). In-game, however, they are depicted as a yellow/green circle with an "!" logo on them.
Use in The Phantom Pain
MB Coins can be used within the main story of The Phantom Pain primarily in order to construct a Forward Operating Base for Diamond Dogs. The first Forward Operating Base is free to construct. Additional FOBs require MB Coins for constructions, however. MB Coins can also be used to accelerate online combat deployments of the Combat Unit as well as hasten the expansion of FOB platforms and development of various weapons and security devices.
Use in Metal Gear Online 3
MB Coins can be used in Metal Gear Online 3 in order to purchase variant camouflage for equipment which is initially purchased through Gear Points, as well as for purchasing certain gear (namely those unlocked on a downloadable content basis).