It means that the coin was produced at the Denver Mint.
Where it was minted. D=Denver
The US did not mint a dollar coin in 1968, therefore you don't have a 1968-D silver dollar.
A 1978-D Eisenhower dollar has no silver in it and is only face value.
SAN FRANCISCO
The 1963-D Franklin half dollar is the highest mintage of the series, most are valued for the silver at about $6.00.
On the reverse above the D in Dollar.
There is no "series D silver dollar" (a coin) so I assume you're referring to a silver certificate. See the Related Question for more information.
The mint mark is under the date, and there is no silver in the coin actually the d is above the year.
The San Francisco Mint
President Dwight D. Eisenhower
Whatsthe value on 1935 A 1935 D, silver dollar certificates..? good condition.
President Dwight D. Eisenhower.