Analgesia is a medical term for pain relief. Stopping pain before it starts is the idea behind preemptive analgesia. In the case of back surgery, this can be done with a single injection of a narcotic mixed with a local anesthetic. It's injected into the epidural space around the spinal cord.
Patients still have some pain, but less than if the pain stimulus was allowed to set off the nervous system. The patient won't be completely numb during the first 24 hours after the operation. Any problems that occur will still cause increased pain and symptoms the patient can report.