If you have the preview pane enabled, you can single-click an item to display it in the preview pane.
or
Double-click the item, with or without the preview pane enabled, to open the item and read it. You can have multiple items in view using this option. However, the contextual options in the main toolbar are only active for the one or more selected items in the list. Whereas, you can take actions on any individual opened items by using its own toolbar. For example, Reply, Delete, view Properties and so forth.