The ATM card is an item in EarthBound. It is a card that allows Ness to withdraw or deposit money via Automatic Teller Machines throughout the world. Money is gained in EarthBound from selling goods at Drugstores or other vendors, and from Ness's father inserting money into Ness's account upon being called by telephone. It is unknown how Ness got his own bank account in the first place, or when it was created.
There is no restriction on the amount of money Ness can carry during the game, however if the party is incapacitated, the accumulated cash-on-hand is divided in half (the cash in the ATM is unaffected). Ness begins his adventure with the ATM card in his inventory, and is restricted from placing it into another party member's inventory, selling it, or dropping it.