Where it was minted. D=Denver
It means that the coin was produced at the Denver Mint.
The mint mark is under the date, and there is no silver in the coin actually the d is above the year.
Please post a new question with the DATE on the coin.
The US did not mint a dollar coin in 1968, therefore you don't have a 1968-D silver dollar.
There is no "series D silver dollar" (a coin) so I assume you're referring to a silver certificate (a bill). However there were different series that went up to "D"; you'll need to check the bill's date and post a new, separate question with that information.
A 1978-D Eisenhower dollar has no silver in it and is only face value.
It is a common date/mintage and in circulated condition it is worth around $10 for the silver in it.
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.
President Dwight D. Eisenhower
This date is the most common of the Morgan silver dollars. The D is the mint mark from Denver were it was made. In circulated condition value runs from $14.00 to $18.00. Uncirculated coins have a retail value of only about $21.00.