Fighters Counters are official markers that are used to keep track of certain numbers on the units on the field. They are simply placed in a fashion that does not obstruct the view of the actual card, but also indicative of which unit gains that number. Previously, fanmade counters were not allowed to be used in the official Japanese tournaments, until it was changed. However, they are still not allowed for the English format.
Counter Unit (3D form)
The Italian branch of Cardfight!! Vanguard has printed their own, unique forms of Fighters Counters known as "Fighters Tokens", that are functionally the same as Fighters Counters, albeit with a different design. These are currently only usable in the Italian format.