system
A chat on subreddit /r/pcmasterrace
human
Undervolting is basically setting the gpu to use a lower voltage at a given clock speed

For example if your gpu runs 1900mhz @ 1.1v, you can undervolt it to run at 1900mhz @ 0.95v.    This reduction in voltage will reduce the power consumption and temps but will keep the same performance as the clock speed is the same.
gpt
If you just set the power limit to 80% the gpu will run  lower clock speeds as it cannot draw enough power to hit the same clock speeds at the higher voltage