(Computerised [Axial] Tomography)
A large doughnut-shaped x-ray machine takes x-ray images at many different angles around the body. These images are processed by a computer to produce cross-sectional pictures of the body, which are then displayed as slices, as if you had been cut up like sliced bread.
This shows soft tissue as well as bones. It may reveal an abnormality of structure, damage, other possible sources of pain such as cancer.
More details of the CT scanning method are given on a page on the Medicine net site.