Article: NVRAM—short for "Non-Volatile Random-Access Memory"—stores settings like speaker volume, the default display, and other settings that your Mac typically uses. Resetting the NVRAM can fix problems wherein your MacBook Pro isn't playing sound, the display is flickering or turning itself off, the computer is taking too long to start up, and similar problems. On some Macs, "NVRAM" is replaced with "PRAM" ("Parameter Random-Access Memory"), which is fundamentally similar to NVRAM. Click the Apple logo in the top-left corner of the screen. A drop-down menu will appear. It's at the bottom of the Apple menu. Doing so prompts your MacBook Pro to shut down. To reset NVRAM, you'll have to hold down the ⌘ Command, ⌥ Option, P, and R keys all at the same time for about 15 seconds. Press the "Power" {"smallUrl":"https:\/\/www.wikihow.com\/images\/5\/5d\/Windowspower.png","bigUrl":"\/images\/thumb\/5\/5d\/Windowspower.png\/30px-Windowspower.png","smallWidth":460,"smallHeight":460,"bigWidth":"30","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of a Windows icon.\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Public_domain\">Public Domain<\/a><br>\n<\/p><\/div>"} button on your Mac to prompt your Mac to begin turning back on. Do this immediately after pressing your Mac's "Power" button; you'll need to be pressing all four keys at once before the Apple logo appears. If the Apple logo appears before you press the keys, you'll have to shut down and try again. Your Mac may reboot itself during this process. Once you arrive at the user selection screen, you can release the keys and sign into your MacBook Pro like you usually do. You may need to reset some of your preferences (e.g., preferred audio output) after resetting the NVRAM. If you are still having issues with the system settings, you may need to factory reset your MacBook Pro. You will lose any data that you have saved on the MacBook if you do so.

What is a summary?
Understand what resetting NVRAM will fix. Open the Apple menu . Click Shut Down…. Click Shut Down when prompted. Find the NVRAM reset keys. Turn back on your Mac. Press and hold the NVRAM reset keys. Continue holding the keys until your Mac finishes starting up. Verify that the problem has been resolved.