macOS Recovery is part of the built-in recovery system of your Mac. You can start up from macOS Recovery and use its utilities to recover from certain software issues or take other actions on your Mac.
How to use macOS Recovery
Turn on your Mac and immediately press and hold Command (⌘)-R or one of the other macOS Recovery key combinations on your keyboard. Continue holding until you see the Apple logo or a spinning globe. Startup is complete when you see the utilities window:
After starting up from macOS Recovery, select from these utilities, then click Continue:
Restore From Time Machine Backup: Restore your Mac from a Time Machine backup of your Mac.
Reinstall macOS (or Reinstall OS X): Download and reinstall the Mac operating system.
The Reinstall macOS utility installs different versions of macOS depending on the key combination you used while starting up. Learn how to reinstall macOS.
Get Help Online: Use Safari to browse the web and find help for your Mac. Links to Apple's support website are included. Browser plug-ins and extensions are disabled.
Disk Utility: Use Disk Utility to repair or erase your startup disk or other hard disk.
These additional utilities are available from the Utilities menu in the menu bar:
To quit macOS Recovery, choose Restart or Shut Down from the Apple () menu. If you want to choose a different startup disk before quitting, choose Startup Disk from the Apple menu.
If you can't start up from macOS Recovery
Newer Mac computers and some older Mac computers automatically try to start up from macOS Recovery over the Internet when unable to start up from the built-in recovery system. When that happens, you see a spinning globe instead of an Apple logo during startup. To manually start up from macOS Recovery over the Internet, press and hold Option-Command-R or Shift-Option-Command-R at startup. Learn more about these key combinations.
If you still can't start up from macOS Recovery, you might need to reinstall macOS after starting up from another disk or volume, or by using a bootable installer.
macOS Recovery over the Internet, the Reinstall macOS utility, and the Get Help Online utility all require an Internet connection. If you need to connect to a Wi-Fi network, move your pointer to the top of the screen, then choose a network from the Wi-Fi 
menu in the menu bar.