It's the mint mark for San Francisco.
The San Francisco Mint
A 1880-S Morgan Silver Dollar that is in a mint state of MS67 is worth: $ 450.00.
Morgan dollars were minted from 1878 to 1904 and in 1921. Please check the back of your coin. You have a Peace dollar rather than a Morgan.
An 1879 S Morgan dollar in MS(Mint State)63 condition is worth somewhere around $64. This is a common date.
If you know it's a 1921 "S" Morgan you found the mint mark
There were 12,760,000 1881-S Morgan dollars minted.
The 1882-S Morgan dollar is common, circulated examples are $26.00-$32.00. A typical Mint State coin is $35.00-$50.00
The last Morgan was struck in 1921. This coin is an EISENHOWER dollar, unless it has a "S" mintmark it's just a big dollar coin. Just spend it.
In MS60 Grade uncirculated, a 1880 S Morgan Silver Dollar is worth about $52. In MS65 Brilliant Uncirculated, it is worth over $330.
If the coin has a Mintmark it is on the back of the coin, above the letters DO in dollar.
6-26-11>>> The 1881-S Morgan is very common, circulated coins are $36.00-$40.00.
Assuming the coin is circulated, the 1921-S Morgan is so common retail values are only $31.00-$40.00.