The circumference of a circle is the length that composes a circle. If you were to undo a circle and lay it down as a flat line that would be the length of the circumference.

You can also explain this relationship of the circumference of the circle with this.

$$c=2\pi r$$

This is going to apply to just about any other circle that you encounter. The $\pi$ value will always be a constant as the circle grows based on the radius.