It's the length around a round object, like if you would run a tape measure around barrel or a ball.
The circumference of a circle is found by the use of a protractor. Placing the point of the protractor at what will be the center of the circle, the point is held steady while the two arms of the protractor are turned around, marking the circle as the protractor turns. When this exercise is finished, the point is in the middle of the circumference, and the circumference figure is measured at any point on the outside circle of the circle, to the point in the center of the circle. This is the measuring of the circumference, from the center point to any other point on the circle. This is done by the study of plane geometry.