This is the bus pass that you can buy at many farmacias and other places that have a sign outside with a picture of this card and the words “Tarjeta Prepago”. You buy the card for $1.70 and then fill it up with however much you want. One bus ride costs 25 cents so if you aren’t taking it every day then $5 will take you far. When you run out of money on the card then you just go into any place with the sign and refill it.
There are other stores with signs outside that say “Prepago” and have pictures of cards on them but these places may just sell the parking meter ticket that you need to fill out and put on your car if you park in certain areas of the city. If you don’t do this then you may end up with a giant ugly sticker stuck to your car window which is actually your traffic ticket.