#preemptions

Television stations in different cities are often run by different companies (sometimes a local company, sometimes a national company), but they contract with major networks to air their programs. So, Channel 4 might be contracted to show CW programming. Channel 2 might show NBC programming, etc. Some large companies own multiple television stations in multiple cities (like Nexstar), but they still give authority to the local station to decide the programming schedule.

So, when a local station decides that a local event is more important than the network provided programming, they'll air that event (like a football game) and preempt (or move) the network programming to a different night or time.