There is a similar question on MSDN - their question was about Meta tags in MOSS 2007 master pages (page description, among others). There, one of the answers was to create a special render control and generate the required HTML code via the control. From the site:
1) Create a render control and override the RenderFieldForDisplay method. For an example, see http://msdn2.microsoft.com/en-us/library/aa981226.aspx. This article shows also how to create the custom edit mode but you don't need all of it.
Deploy the custom control as mentioned in the article.
2) Add the contentplaceholder in the master page, or use the default
Place this in the section, right after the tag for example.
3) Call the custom control to render the meta description field in the page layout:
<asp:Content ContentPlaceholderID="PlaceHolderAdditionalPageHead" runat="server">
<PublishingWebControls:EditModePanel runat="server" id="displayModeMetadata" PageDisplayMode="Display" SuppressTag="true">
<Mytag:HeadMetadataFieldControl FieldName="MyMetadatafieldname" runat="server"></Mytag:HeadMetadataFieldControl>
As far as
ContentPlaceholder goes, Microsoft explains it here. There, it is said:
If you want your changes to a content placeholder to apply to the entire site, modify the content placeholder on the master page. You can also modify the content on an individual content page, thereby overriding the default content that is supplied to that content page by the master page.
Therefore, if you wish to edit/add the description, you have to do it by editing the content placeholder.
There is a Microsoft's article on creating publishing pages. There it says that there is a Description column in Pages Document Library, and, if it isn't shown, it can be added there like a regular column. You can then try to edit that column, like a regular one.
Enter a description in the Description box. This text is stored in the Description column of the Pages document library. By default this column does not appear in the library, but you can add it like any other column.