what does it mean when they say circumfrence?

They mean the outside of a circle.

\(C=2\pi*r \)

Where r is the raduis

Circumference is the length around the edge of a circle

Circumference: the enclosing boundary of a curved geometric figure, especially a circle.

It is the distance around the edge of a circle (or any curvy shape). 

It is a type of perimeter.

Also, the equations to find the circumference are:

\(C=\pi*d \)

AND

\(C=2*\pi*r\)