You can access your Portal Page(s) through Page Manager. Portal pages are listed at the bottom of the list of site pages; click on the + sign to expand the "Portals" section and display individual portal pages.
This will open up the portal page in Page Manager, where you can adjust some of the basic settings for that page. To edit the configuration of the various portal elements, click on the "Unpublished" tab. This will put you into "Administrative Mode," where you can add and remove elements, and configure the elements that are already present on the portal page.
In Administrative Mode, you can drag-and-drop elements around on the portal page to rearrange them, or add new elements to the page. Click the "Add Elements" button in the lower-right corner to open up the Element Library, then drag new elements from the Library onto the portal page. It is possible to create more than one of the same type of element - for instance, you could create two "YouTube" elements, each one pointing to a different YouTube video.
The yellow-and-black striped line separates "docked" elements from regular elements. A "docked" element appears on the portal page by default, and cannot be moved or edited by individual users. All users who view the portal page will always see docked elements. To dock an element, drag it so that it is positioned at the top of a page column, above the striped line. Elements that are not docked can be removed or edited by individual users - this way, users can customize the various elements on a portal page to their specific liking.
The same holds true for tabs on a portal page - drag a tab to the left of the "Docked Tabs" indicator to ensure that it will always appear on the portal page for all users. Tabs to the right of the "Docked Tabs" indicator can be edited or removed by individual users.
You can adjust the settings of any element or tab by hovering over it to reveal the yellow pencil icon. Click on the icon to open up a new window, where you can configure the properties of that tab or element.