A silhouette is the outline and dark shape of a person or object against a lighter background.

If your camera is in an automatic mode (P, Av, Tv), it tries to make sure you can see everything, and will probably adjust the exposure so that there is no silhouette. Put your camera in Manual mode (M), and set your exposure based on the background.

Remember: the viewer should be able to recognize the person or object from the silhouette.