If you have access to more than one calendar in Outlook e.g. shared calendars you can change how these are displayed.
The Overlay view places all calendars one on top of the other so you’re presented with an aggregated view – similar to looking at one calendar with information from all shared calendars visible together. Each calendar appears as a tab at the top of the screen, each with it’s own colour option and the appointments from all calendars will be displayed in each calendar in the corresponding colour.
If you prefer to view the calendars side by side, click the arrow to the left of the calendar name. You can click this for each calendar to view them all side by side or for just one if you want to leave all other calendars in Overlay mode.