Panning is a creative technique that allows you to show a moving subject in focus but convey the sense of movement with a blurred background. Lock your focus when you frame the subject.
Then as the subject moves, keep her in the frame and press the shutter release. Be sure to keep up with the moving subject. When I tried photographing my daughter on the swing without panning, above, she was a blur. With panning, I caught her in focus and the background was blurred, which gives the shot, left, more energy.
It takes practice to keep your camera trained on your subject -- particularly if she's moving fast. Follow the subject in one smooth motion by twisting just the upper part of your body.