the timing tab on the cover corresponds with a line on the dampner. when the spark plug fires, with a timing light attached, it flashes. when you point it at the dampner, the marks on the tab shows you how advanced or retarded your timing is.
example, say when the light flashes, when the mark on the dampner is at the 0* tdc mark on the tab. that means that the spark is going off when the piston is tdc. spin the distributor housing, and you can advance or retard it. meaning you can make it spark before or after it reaches tdc. to get the car started, set the distributor is so that the pointer wuold be right before the wire that leads to the number 1 cylinder.