Pennies are essentially made from copper(and a few more metals), and after a period of time the copper oxidizes to form copper oxide. This gives the penny a dull blackish color. Copper in it’s natural form is actually a bright and shiny.
When you mix salt and vinegar and drop pennies into the solution the acetic acid from the vinegar will react causing the copper oxide to dissolve. Hence, you are left with shiny clean pennies.
To clean the pennies, you will have to leave the pennies in the solution for about five minutes.