Basically, this is a Smart Card similar to those found in Singapore or Hong Kong where you can touch in and touch off every time you board a bus, tram, or train.
It is also worth noting that this is the only way to pay for public transport when commuting around the city. It's light, durable, and can be bought from a myki machine (full fare myki only) at all metropolitan train stations, even at 7-11 convenient stores.
The card costs 6 AUD with no value. Top it up for your travel.
