L5S1 refers to the spinal disc that connects the 5th vertebrae in the lumbar spine to the 1st vertebrae of the sacral spine. This region is also known as the L5S1 motion segment, and helps to distribute loads from the spine to the legs and pelvis.
A spinal disc is a piece of soft tissue that sits between the vertebrae in your spine. They act as shock absorbers when you do things like walk or run, and they aid in spinal mobility, similar to what cartilage does in other joints. Given the importance of things like walking and running, these discs play a pretty important part in helping us get around, though most people may not even know this until they injure or damage one.