The gauge of a wire is a measure of its thickness. Confusingly the lower the gauge of the wire the thicker it is. A 12 gauge wire is thicker than a 20 gauge wire.
In this country we refer to the thickness of wire in millimetres, whereas in the USA they refer to it by gauge. I often find it hard to flip between the two when I am reading American bead magazines so I use the wire gauge conversion chart below.