The Activate a Specific Application action switches you to the specified application.
You can optionally bring all windows to the front, or only one window. You can optionally ask the application to open its initial window (eg an Untitled document).
If the application is already at the front, you can choose to leave it there, or hide or quit it, or switch to the previous application.
You can (v9.0+) select whether to match the application in the normal way (based on the path or the bundle ID), or require the path, or just by the bundle ID.