Sube is the name for the (new) bus payment system in Buenos Aires. It means an end to carrying a small purse full of coins.
Public transportation is plentiful. The trains leaving retiro (trainstation) to the suburbs of Buenos Aires are brand new. The public bus system works great: they come often and remain very affordable (less than $0.50 for a trip).
Finding a sube card is a challenge in its own. We visited several corner stores (kiosko) to buy a sube card. When we eventually found a store which sold sube cards, we then had to go and visit yet another store to load the card with money.
How a tourist is supposed to figure this all out is beyond me.