You may have heard that resetting the NVRAM can fix all sorts of problems. This isn’t the case, but it can fix a few issues. NVRAM is where Mac OS X stores information like your settings for speaker volume, your screen resolution and startup disk info. So resetting your NVRAM will only fix issues like sound not working, screen resolution problems, and slow startup times.
If you’re experiencing any of those problems, you can reset your NVRAM by powering on your Mac while holding down the Command + Option + P + R keys until you hear the startup chimes sound TWICE.
After you hear the second startup chime sound, you can release the keys and allow the Mac to boot normally.