It's called a mint mark and means the nickel was minted in Denver, and was made in 1964 or earlier.
US Coins dated 1965-67 didn't carry mint marks due to the so-called Great Coin Shortage of the 1960s. Mint marks were resumed in 1968 but were moved to the front sides of coins, usually near the date.
Other possible mint marks on modern coins include
monticello
The nickel. Monticello was Thomas Jefferson's home so it nicely complements his image on the front of the coin. Of course, if you look at the nickel you'll see that the building has the name MONTICELLO right underneath the picture!
With no mint mark next to Monticello, or a "D", it's worth about a dime assuming average condition. If there's a small "S", it's worth about a quarter.
1951 is a common date. In average condition most are worth 10 to 30 cents. A 1951 nickel with an S mint mark next to Monticello would retail for about 50¢ if it was only slightly worn.
It's Monticello, Thomas Jefferson's house. The name is written directly under the image.
monticello
buffalo
The nickel. Monticello was Thomas Jefferson's home so it nicely complements his image on the front of the coin. Of course, if you look at the nickel you'll see that the building has the name MONTICELLO right underneath the picture!
Monticello
With no mint mark next to Monticello, or a "D", it's worth about a dime assuming average condition. If there's a small "S", it's worth about a quarter.
1951 is a common date. In average condition most are worth 10 to 30 cents. A 1951 nickel with an S mint mark next to Monticello would retail for about 50¢ if it was only slightly worn.
The US nickel (five cent coin) has an image of Jefferson's home, Monticello, on the back.
It's Monticello, Thomas Jefferson's house. The name is written directly under the image.
The US 5 cent coin (a nickel). That is his home- Monticello- on the back.
The presence of the word Monticello under the building indicates it's an image of Thomas Jefferson's home on the other side of a nickel.
Monticello is the home of the president shown on the coin's front side, Thomas Jefferson.
US nickels show a picture of Thomas Jefferson's home, Monticello, but they're not minted there. At that time there were 3 active mints: Philadelphia (no mint mark), Denver ("D"), and San Francisco ("S") although 1950 nickels weren't minted at the latter. Please see the question "What is the value of a 1950 US nickel?" for more information.