Cauda equina syndrome is compression of the central tube of nerves below the lower end of the spinal cord (usually L1) including compression of the nerves to the bladder and bowel.
Typical symptoms include low back pain, sciatica (often in both legs) and altered sensation around the back passage and genitals. Bladder symptoms include: loss of bladder sensation, abnormal sensation on passing urine, urinary retention (more common in men), urinary incontinence (more common in women). Bowel symptoms are rare but include incontinence.
This is an emergency because the nerves to the bladder and bowel are very sensitive and once compressed and producing symptoms, permanent damage may result if they are not decompressed urgently. The long term results for bladder and bowel function are worse if the nerves are decompressed more than 48 hours after the onset of bladder and bowel symptoms.