To open a file, you want to visualize, click on its name in the File panel and then click on the Open button. You can also open files by double-clicking on the file in the File panel. Once a file is open, the Open button turns into the ReOpen button and the file can be visualized. If you click on a file that has been opened before, the Open button becomes the Activate button. Clicking on the Activate button causes the selected file to become the new open database, from which plots can be created.
When the ReOpen button is clicked, all cached information about the open database is deleted, the database is queried again for its information, and any plots that use that database are regenerated using the new information. This allows VisIt to access data that was added to the database after VisIt first opened it.
Virtual databases, unlike ".visit" files, are expanded in the File panel by default because the time states are usually known before the database is opened. Since the time states are known before the database is opened, and you can click on any of them, it is possible to highlight a later time state of a virtual database and open it at that later time state. Opening a virtual database at a later time state can be useful if the database has variables that are introduced later in the time series.