Ground Stop is simply a stoppage of all flights into a certain area. If you are traveling to an airport that has become so congested with air traffic that it needs to clear out some space before new aircraft can depart for that airport, they can issue a 'ground stop.
A ground stop is like your sink filling with water, even when your drain is open. You can watch the water going down the drain while simultaneously filling up. You need to slow the water, or turn it off before you can lower the water level in the sink.
Other definition:
A Ground Stop is simply a stoppage of all flights into a certain area. If you are traveling to an airport that has become so congested with air traffic that it needs to clear out some space before new aircraft can depart for that airport, they can issue a 'ground stop.'