Office 365 (Outlook on the Web) - Reading Pane Overview

The reading pane is where messages are displayed. You not only can read messages in the reading pane, but also respond to them without opening a new window.

Reading Pane

To respond to a message, select Reply at the top of a message or click on the drop-down arrow next to it for other choices such s Reply all or Forward. A new message will open appears in the reading pane.
Add content to the body of the message and use the options across the top of the window to Send, Attach a file, or Discard.
To view more options such as Insert signature, Check names, Set importance, etc., select More for an extended menu.

To change the location of the reading pane

On the nav bar, select Settings > Display settings > Reading pane
Select one of the following:

Show reading pane on the right (this is the default setting)
Show reading pane at bottom
Hide reading pane

Select OK to save your selection and to close the Display settings window.