Ligaments are the tough bands of tissue that hold the vertebrae together and keep the spine stable as it moves. The main ones are the anterior and posterior longitudinal ligaments which run the length of the spine at the front and back of the vertebral bodies, the ligamentum flavum which lines the back of the spinal canal and the interspinous and supraspinous ligaments, which connect the bony projections you can feel down the middle of your back. A ligament injury, often called a sprain, happens when one of these bands is overstretched or torn, usually by a sudden force. Most spinal ligament injuries are minor, settle with time and the right care, and never need surgery.
Ligaments matter because they are part of what keeps the spine stable. A minor sprain heals, and the spine stays stable. A more serious tear, or long-term wear that stretches the ligaments, can leave a segment of the spine moving more than it should. That instability is what occasionally needs more than rest.