A voltage regulator normally takes a higher voltage and transforms it into a lower voltage.
The reason to install a voltage regulator is so that you don’t have to power it via the USB or 5V input.
In other words, to run Smoothieboard standalone via the Panel or Ethernet.
Smoothie’s power input is 12-24v, and the voltage regulator takes it down to 5V.
(see the “More Power” section before purchasing the part):
R-78E5.0-0.5
Which can be purchased from Robotseed or Uberclock
R-78E5.0-1.0
is the same size/pin configuration as the recommended part above.This is how it looks when soldered on:
Red circle marks where it is located
Note the orientation