In order to give you an intuition of what "Program Slicing" really does, I will show you an example.
The procedure SumAndProduct calculates the sum and the product of a vector of numbers.
During program maintenance or reverse engineering, we might want to localise bugs or we might simply want to understand the program.
Program understanding can be greatly simplified if we can identify independent parts of the program. In this case, the calculation of the sum and the calculation of the product are independent of each other.