This is a novelty coin not made by the US mint and has no collectible value.
The US Mint did not issue an 1896 nickel with an "S" stamped on it. If you have one then it would seem as though someone stamped the coin sometime after it left the mint. Also please don't assume that because a coin is old it has to be made of silver. All US nickels except special "war nickels" made during 1942-45 are struck in the same alloy of 25% nickel and 75% copper.
Mintmarks identify which mint the coin was made at.
The letter is known as the mint-mark and tells where the coin was made. A coin having a P mint-mark (or no mint-mark on some coins) was made in Philadelphia, a coin having an S Mint-mark was minted in San Fransisco, a coin with a D mint-mark was minted in Denver.
It means the coin was not issued by the U.S. Mint and is just a copy having no monetary value.
Coins made at the Philadelphia mint did not have mint marks until the 1980s. Your coin was made at the Philadelphia mint.
The letter "D" is the mint mark for the US Mint in Denver, Colorado.
For coins dated 1906 to the present, it means the coin was made at the Denver Mint. A "D" mint mark on an old (1838-1861) gold coin means it was made at the long-closed gold coin mint at Dahlonega, GA.
Not by the US Mint.
If it's a small letter below the date, it's a mint mark indicating the coin was struck (not "stamped", btw) at the San Francisco mint. S-mint coins are sometimes worth a bit more than ones from Philadelphia or Denver, so you might want to post a new question with the coin's date. If it's located anywhere else it was added by someone after the coin was struck, and the coin is only worth a penny because it has been damaged.
There is not always a mint mark on coins. If your coin does not have a mintmark it means that it was made at the Philadelphia mint in Pennsylvania.
A mint-mark on a coin is a letter designating the location of the mint where the coin was made. "D" for Denver, "P" for Philadelphia, "S" for San Francisco.