The diameter is the distance across the circle, so a 6 inch diameter would be 6 inches from one side across the middle to the other side. Most ladies hands, from heal to tip of middle finger, is 6-7 inches so you can use that as a visual. I hope this helps. (The other answer was for the circumference rather than the diameter.)
Pinch your finger together with your thumb to form a circle and this is approximately 6 inches around.