How to reset a Mac's NVRAM, PRAM, and SMC
When your Mac starts acting up, you’ll probably run through some common troubleshooting procedures, such as restarting it, running Disk Utility, and perhaps performing a Safe Boot. Your repair repertoire should also include a couple of additional procedures that can occasionally eliminate otherwise inscrutable problems—zapping the NVRAM and resetting the SMC.
Zap the NVRAM (or PRAM)
Back in the day, the standard list of quick fixes for random Mac ailments always included “zap the PRAM.” The P in PRAM stood for parameter (the RAM was just RAM—random access memory), and it referred to a small amount of special, battery-backed memory in every Mac that stored information the computer needed before it loaded the operating system. If the values in this memory got out of whack for one reason or another, your Mac might not start up correctly, or might exhibit any of numerous odd behaviors afterward. So you could press a key sequence at startup to reset (or “zap”) the PRAM, returning it to default, factory values.