A ground stop is an air traffic control measure that slows or halts the flow of aircraft that meet certain criteria, requiring all aircraft of a certain criteria to remain on the ground.
This allows, in this example, for Newark to deal with the task at hand before preparing for arriving aircraft once the ground stop is lifted.
Ground stops may occur during an operational event (e.g., a computer outage), a thunderstorm or other weather concerns (e.g., due to the danger of wind shear or hail), or another hazard.
Additionally, pilots and flight crew may be unable to reach their assigned aircraft on time, leading to further disruptions.
As a result, subsequent flights may also be delayed or canceled due to crew members being unavailable.