From the AIM at: http://www.faa.gov/ATpubs/PCG/pcg-m/pcg-m.html
METERING- A method of time-regulating arrival traffic flow into a terminal area so as not to exceed a predetermined terminal acceptance rate.
Note that at the AIM site above, there are several other metering-related definitions.
In a general sense, the concept of metering aircraft is not unlike the concept whereby some freeway entrance ramps have traffic lights that tell you when to enter the stream of (heavy) traffic. In the case of aircraft, a destination airport's acceptance rate is often affected by weather and related operational restrictions, which, back to auto anologies, is akin to closing 1 or 2 (or more) lanes of a 3 or 4 lane freeway. The big difference between the two transportation modes is that aircraft can NEVER be allowed to get "bumper-to-bumper" in the air, thus traffic flow must be regulated to keep it flowing while keeping it properly separated.
In your ORD scenario, some of the destinations served by airlines there were most probably suffering from reductions in their acceptance rates, and traffic to those destination was being delayed to await their "green light" to enter the airspace system. The ATC metering position essentially keeps track of when (as dictated by FAA ATCSCC in Washington) various aircraft will be allowed to takeoff.
Flights destined to LAX, SFO, and SEA commonly experience these types delays should cloud ceilings and/or visibilities there slow the flow of arrivals into the airport (by precluding the use of visual approaches in favor of ILS approaches). Flights destined to these, and just about any other airport can also (and unfortunatly, do also) occur should thunderstorms be slowing arrivals at the destination airport, or even the route between the departure and destination airports.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.