Reply #11: From the FAQ

[Skinner]

What do "Mark" and "Mark All" do?

The "Mark" function helps you keep track of which posts are new. When you click "Mark" in a particular forum, all of the posts in that forum will get marked with a white icon, like this: . Any messages that are posted in that forum after you click "Mark" will be marked with a gold icon, like this: .

The "Mark All" icon marks messages in every forum.

Note: In order to use the "Mark" function you need to have the function turned on in your preferences. Click on "Options" then click "Edit your preferences" (or just click here). Under the section "Use MARK time stamp feature?" Select "yes" and then click the "Update" button.

http://www.democraticunderground.com/forums/faq.html#mark