it is a transponder (almost size of a cigarette pack) that you preload with a certain amount. used to pay cash less toll fees on expressways/highways. whenever you arrive at a toll booth, the sensor detects your ezpass and lets you in by lifting up the boom, instead of paying cash. the system then deducts your preloaded credits by the amount you have to pay for using the expressway. very convenient as the queue for ezpass lanes are short and faster to get through
i'm sure there is a similar system in other countries but just call it differently