Private jets (like commercial airliners) are made to be in the air or waiting at a regional airport for as short a period of time possible for its next "leg".  An empty leg is when the jet Charter  is not in use due to weather, lack of bookings, maintenance, cancellations, delays, or similar inability to get from one place to another.

An empty leg is expensive.  Very expensive.  If an aircraft is booked fully and properly, an unexpected empty leg affects the next leg, the next leg, the next etc.  In that case, there is a scramble to get a plane of the same class (type) for the next few day's and week's private flyers because the plane that is expected to be available for them going forward is sitting on an airstrip with a hundred or so others somewhere near Kentucky.