What a circular mil is
A circular mil is the area of a circle one thousandth of an inch in diameter. It is a convenient unit because area in circular mils is simply the diameter in mils squared — no π required. Conductor area is what actually determines resistance, which is why voltage drop calculations work from circular mils rather than from AWG numbers.
The AWG scale runs backwards, and roughly geometrically: every three sizes down approximately doubles the area. 14 AWG is 4,110 circular mils; 11 sizes larger, 4/0 is 211,600 — over fifty times the copper.
DC versus AC resistance
These are DC values. On AC circuits, skin effect and conductor reactance add to the effective impedance, and the difference grows with conductor size and frequency. For small branch circuits the DC figure is close enough. For large feeders, AC impedance figures give a materially better answer than the circular-mil estimate used by this site's voltage drop calculator.
Resistance also rises with temperature — a conductor running hot has more resistance and drops more voltage than these figures suggest.