Wheelbase is a measurement used to describe the placement of wheels, usually for a vehicle of some sort (automobile, bicycle, skateboard, etc.)
The measurement is shown on technical drawings as the distance between the points where the wheels meet the ground. It should be noticed that this distance is identical to the distance between the respective centers of the axles upon which said wheels turn.
The general convention is that the longer the wheelbase, the more stable the vehicle. However, the turning radius increases with wheelbase, also, so maneuverability is lost.
Example: A car with a 168-inch wheelbase (unrealistic, I'm aware) would be much more stable than one with an 84-inch wheelbase. However, the car with the 14-foot wheelbase would have a little trouble cornering.