What does the letter over Monticello on a U.S. nickel mean?

In mid-1942 the composition of the nickel was changed from a copper-nickel alloy to a mixture of silver, copper, and manganese because nickel metal was needed for the war effort. To distinguish these coins from ordinary nickels, the Mint decided to move the mint mark letter from the side of Monticello to a position above the dome, and to use a much larger letter. In addition, a "P" was used for Philadelphia coins for the first time in history.

In 1946 regular copper-nickel coinage was resumed, the mint marks went back to their then-usual position beside Monticello, and the use of a P on Philadelphia coins was discontinued. No new coins would use a P mint mark until 1979 (dollars) and 1980 (all other coins except cents).