In the Netherlands all Public Transport (Trains and Busses) can only be used when the passenger has in his possesion an OV-chipkaart which allows him to enter the Bus or the Railwaystation by having this card scanned with the scanner in the Bus or at the entrance of the Railway station. At the larger Railway Stations the entrance to the platforms is blocked with a turnstile gate
Sometimes a Pedestrian way is running through the railway station, connecting two parts of the city/village. Because the entrance to the station (and hence the Pedestrian way) is locked with a Turnstile Gate that can only be opened with the OV-chipkaart, persons who only want to use that Pedestrian way to get from one side to the other (they don't need nor want to get to the train platforms), still need the OV-chipkaart to open that gate. This is called checking-in. If they Check-out at the other end (within a certain timeframe), their OV-chipkaart will not be charged.