What does 1942 D mean about a us quarter?

The D is a mint mark, indicating that the coin was struck in Denver.

Other mint marks on modern coins are

S = San Francisco, on circulating coins up to 1955 and occasionally from 1968 to 1975, and on proof coins from 1968 onward.

P = Philadelphia, on coins from 1979/80 onward, and 1942-45 nickels. Coins without a mint mark are also from Philadelphia.

W = West Point, on commemorative coins only.