Gmail is no doubt the best mail service on web for free, and with new upcoming features gmail team make gmail more interesting product day by day. For those who are used to using outlook reading pane, now you can get the same experience in gmail web interface.
Just like you see in outlook reading pane, you and preview items your Inbox without opening them by just click the message to display the text of the item in reading pane, same you can now do in gmail as well, you can now preview messages in your inbox using a new feature in Gmail Labs called Preview Pane.
In order to turn on gmail preview pane either horizontal or vertical, you will need to navigate gmail labs section under settings as shown in the image below.
Go to gmail mail settings and then click labs tab under gmail settings
In labs features, scroll down the page and search for preview pane, simply search using ctrl+f to search on it on the current page and enable this feature.
Once you enable this feature click on save changes button at the bottom of the page or at the top of the page.
Now you will see the preview pane in your inbox as shown in the image below.
You can either enable the preview pane in vertical mode or horizontal mode as per you like it, simply disable it through the dropdown shown on the top right above the inbox mails list as shown in the image below.
Or toggle between different modes