If you break down the cam numbers they tell you at which crank position the valves open and close.
For example the 440 HP cam spec is 268/284 advertised duration (I believe rated at 0.006" lobe lift.) Lift is rated at 0.450"/0.458" at the valve assuming a 1.5:1 rocker ratio. The actual cam lobe lift at the tappet is figured by dividing the advertised lift by the rocker ratio.
Anyhow, the cams advertised duration along with the lobe seperation angle determines the amount of overlap the cam has from the point of the intake opening to when the exhaust closes.
In this example the cam had a 115 LSA resulting in 46 degrees of overlap (again using the 0.006" lobe lift numbers.)
The Installed centerline indicates the crank position when the intake lobe is at maximum lift (or the center of the lobe.) In this example the cam is installed with a 113 degree intake centerline to get the valve events described in the Factory Service Manual of:
Intake Opens at 21 degrees Before Top Dead Center.
Intake Closes at 67 Degrees After Bottom Dead Center.
Exhaust Opens at 79 Degrees Before Bottom Dead Center.
Exhaust Closes at 25 Degrees After Top Dead Center.
The overlap is just adding the Intake open at 21 plus the exhaust close at 25 = 46 degrees.
If you change the cams installed centerline to 110 degrees, the timming events are 24/64/82/22.
This "Advancing" of the cam males the intake open sooner before TDC, but close sooner after BDC trapping more compression pressure. On a stock 440 HP this should increase cranking compression pressure about 5 psi. Because the overlap is "ground" into the cam it remains the same reguardless of installed position because it is directly related the the cams lobe seperation and duration.