Why must the pipette be washed with the concentrated solution?
Water still remaining in the pipette might dilute the concentrated solution.
Why can the graduated flask be washed with distilled water?
That does not change the number of moles of the solution which will be introduced.
Where does the meniscus come from?
By capillarity the solution climbs up the walls of the lask.
Explain the error of parallax !
If the graduated flask is seen from above, the lowest point of the meniscus is not detected correctly.