Spondylolisthesis, or the forward slip of a vertebra over the one beneath it, involves several main parts of your vertebrae. First off, the vertebrae are the bones that make up your spine. Most people have 33 vertebrae in their spinal column.
Those 33 vertebrae are divided by region: your neck (cervical spine), mid-back (thoracic spine), and low back (lumbar spine). At the lower end of your spine, you also have the sacrum and the coccyx, which is commonly called your tailbone. Spondylolisthesis usually happens in your lumbar and sacrum regions.
On the vertebra, here are the structures that you need to know in order to understand spondylolisthesis:
Between each vertebra, you have an intervertebral disc. It works as a cushion, absorbing shock as you move, and it allows you to move your spine in multiple directions. There are two parts to the disc: the center part is called the nucleus pulposus, and the outer part is called the annulus fibrosus. Think of your disc as a jelly donut (it'll help you visualize the structure). The nucleus pulposus is the jelly; it's made of a gel-like substance and is the part that acts as a shock absorber. Around the "jelly" is the tougher annulus fibrosus, which holds the nucleus in place.
The annulus fibrosus and the nucleus pulposus are both made of collagen, water, and proteoglycans. However, the nucleus has more water and proteoglycans—more fluid—than the annulus, and that's what gives it its gel-like characteristic.