If you're a member of more than one community, those communities might be connected to each other. If they are, while you're in one community you can see into the other through widgets. Communities are connected to each other by a community administrator. If your communities aren't connected, you won't see ways to view content from one inside another.
When you're personalizing your home page, be sure to look to see whether there are tabs of widgets for other communities you're a member of. For more information, see How can I personalize my view of content?