AWG, short for American Wire Gauge, is a standardised scale for describing the thickness of electrical wire. It is commonly listed in the specifications of USB cables, particularly those intended for higher power charging, because wire thickness affects how much current a cable can carry efficiently.
Confusingly, a lower AWG number means a thicker wire, so a cable listed as 20 AWG has thicker power wires than one listed as 28 AWG. Thicker wires have less electrical resistance, which reduces voltage drop and heat over the length of the cable, and matters most for longer cables carrying high current for fast charging.
For everyday charging and short cables, AWG differences rarely matter much, but for longer cables used with high wattage Power Delivery charging, a thicker gauge helps maintain full charging speed without excessive heat. See our guide to USB cables for buying advice.