Gmail Preview Pane is another new feature in Google Gmail Labs. Inspired by Gmail on tablets, Preview Pane split Gmail into 3 panes, and works best for you who have a big monitor.
Preview Pane is not turned on by default. You can enable it in Gmail Labs. After enabled, you will see a toggle button in the top right corner of your message list. There are 3 options: No Split, Vertical Split, Horizontal Split.
Vertical Split shows message preview below message list, whereas Horizontal Split shows message preview next to the message list (best for wide screen monitor).
Preview message will be marked as read after 3 seconds. You can change the timing in the Gmail settings.
My two cents
Though Google means “Preview”, Preview Pane is actually full feature like you open a message in Gmail. Using Preview Pane is more productive than traditional Gmail message list because you do not need to flip between email list and email messages.
Highly recommended! Use Preview Pane if you have a larger resolution screen. (I’m using 24-inch monitor)