Are you talking about a Portal page, or ASPX template? With ASPX Template you should not see the layout tab, as the template and all web parts are defined in a physical ASPX file.
With Portal page template the layout defines web part zones, in which you can then place web parts in the Design tab of the Document you are editing. Those web parts are part of the template and will be available on any document using that template.
If you are using any Editable web parts and filling out their content in the Page tab, then this content is not part of the template, it is so called Page-specific content
. This allows content designer to predefine document Design and leave filling out the content of each page to the content editor.
If you wish to add a text or other content to the template which should show on any document using the template, use Static text or Static HTML web part. Alternatively, you can also enter Default text in the Editable text web part properties which will show up if no other content is entered in the Page tab.
You can find more information about how templates work in our Portal page