What is SMC,PRAM and NVRAM?

SMC - The system management controller - a chip found in Intel-based Macs. PRAM (parameter random access memory) and NVRAM (non-volatile random access memory) holds Mac configuration information.

When you would need to reset SMC,PRAM,NVRAM?

Well if you experiencing following:

• The battery or status lights behave strangely
• The keyboard backlight doesn’t work properly
• Your MacBook doesn’t turn on when you open it
• The power adapter light doesn’t reflect what it’s doing
• The fan is running at an abnormally high rate, especially under low load
• The trackpad doesn’t work
• Your computer won’t connect to Wi-Fi
• Target display mode switches on or off unexpectedly
• App icons bounce for an extended period of time when opening
• Your computer runs slowly, even under low CPU load
• Volume doesn’t respond properly
• The boot volume isn’t set
• Strange mouse scrolling and clicking speeds
• Keyboard not responding normally
• The time zone or clock is wrong
• Display resolution changes or won’t change
• AirPort problems
• Your computer shuts down slowly

is good sign that you need one of the following (or both) steps to try.

Actual reset procedure

SMC Reset: Hold Shift + Control + Option on the left side of the keyboard and press the power button (at the same time) hold them all for 10 seconds then release them and start the Mac

PRAM/NVRAM Reset: hold the Command + Option + P + R keys while pressing the power button, hold all the keys and wait for the Mac to start and then restart, then release the keys.