Voltage drop over a copper wire run, from gauge, one-way length, current and supply voltage.
Current has to travel out to the load and back through the circuit, so the effective conductor length is twice the one-way run. The calculator asks for the simpler one-way distance and doubles it internally.
Each AWG gauge has a different resistance per 1000 ft of copper — thinner wire (higher AWG number) has higher resistance per foot, so it produces more voltage drop for the same length and current than a thicker gauge.
The National Electrical Code informally suggests keeping voltage drop to 3% or less for branch circuits and 5% or less combined with feeders, as a comfort/performance target — not a strict code violation. The status label just flags where your result falls against those commonly cited figures; always confirm against your local code.