Penis circumference is the same thing as penis girth. It is the thickness of the penis.
Average Penis Circumference
Measuring Penis Circumference
To measure the penis circumference, you can use a soft tape measure (simply wrap around the penis, probably in the middle) and record the number. You can also wrap something like string around the penis, mark the point that encircles it, and then measure that with a straight ruler.
Increasing Penis Circumference