Like a toll gate to collect the zone fee from the foreigners to enter the Bagan Archaeological Zone. By paying the fee 25,000 MMK you will get the card which will allow you to enter most of the temples (permitted areas, mentioned in the card or you can ask the gate in-charge persons), valid for 5 days. Better keep those cards safe and bring them wherever you go in Bagan.
If you are by taxi or have your own motorcycle, just take any other road into bagan to circumvent purchasing a ticket here for 25000. If you do so, please spend a bit more buying from the kids in the pagodas.