It means the launch has to happen at that exact moment, otherwise the path of the spacecraft in orbit will be too far away from the space station for a rendezvous to be possible.
From what I heard, they have multiple such launch windows, each about an hour and a half apart (each corresponding to an orbit of the earth by the ISS)
From what I heard, they have multiple such launch windows, each about an hour and a half apart (each corresponding to an orbit of the earth by the ISS)