Reading pane is the function integrated from Microsoft Outlook. Many users use it as it allows them to view the mail without opening it.
This function is available in Gmail too.
1) You have to select the gear icon at right side in your gmail.
2) There you will find Labs.
3) By clicking on Labs you see a list of function which can be enabled in your gmail account.
4) In that you will find Preview Pane option, enable it and click on Save Changes.
5) By refreshing you will see a dropdown icon beside the Gear icon of settings.
6) By selecting the type of preview you want you will be able to see a preview pane in your Gmail window.