Browse privately in Safari on Mac
When you browse privately, the details of your browsing aren’t saved, and the websites you visit aren’t shared with your other Apple devices.
Browse privately one time
In the Safari app
on your Mac, choose File > New Private Window, or switch to a private window that’s already open.A private window has a dark Smart Search field with white text.
Browse as you normally would.
When you use a private window:
Browsing initiated in one tab is isolated from browsing initiated in another tab, so websites you visit can’t track your browsing across multiple sessions.
Web pages you visit and your AutoFill information aren’t saved.
Your open web pages aren’t stored in iCloud, so they aren’t shown when you view all your open tabs from other Apple devices.
Your recent searches aren’t included in the results list when you use the Smart Search field.
Items you download aren’t included in the downloads list. (The items do remain on your computer.)
If you use Handoff, private windows aren’t handed off to your iPhone, iPad, iPod touch or other Mac computers.
Changes to your cookies and website data aren’t saved.
Websites can’t modify information stored on your Mac, so services normally available at such sites may work differently until you use a non-private window.
Note: None of the above applies in non-private Safari windows you may have open.
Always browse privately
In the Safari app
on your Mac, choose Safari > Settings, then click General.Click the “Safari opens with” pop-up menu, then choose “A new private window”.
If you can’t see this option, choose Apple menu
in the sidebar, then turn on “Close windows when quitting an application” on the right. (You may need to scroll down.)
Stop browsing privately
In the Safari app
on your Mac, close the private window, switch to a non-private Safari window, or choose File > New Window to open a non-private window.Do any of the following to further enhance privacy:
Delete any items you downloaded while using private windows.
Close any other private windows that are still open to prevent other people from using the Back button
or Forward button 
to see web pages you visited in those windows.
Besides using private windows, you can manage cookies and data stored by all websites and prevent cross-site tracking.
If you forget to use a private window, you can clear your browsing history.