A launch window is arithmetic dressed as suspense. It is the span of time during which a rocket can leave the ground and still reach the orbit it was designed for.
Instantaneous windows
Missions that rendezvous with something already in orbit — a station, a departing planet — often get a single instant. Miss it and the geometry no longer closes.
Open windows
Satellites heading to a generic low Earth orbit can often fly any time the weather allows, which is why those launches get scrubbed and recycled the same evening.
