Basically, it is one vertebra in your back slipping over the one below it due to a number of factors. This is the down and dirty explanation from Google:

Anterolisthesis is a spinal condition in which one vertebra (a bone in the spine) slips forward over the vertebra below it. This displacement can occur in any part of the spine, but it is most common in the lumbar (lower back) region.

Causes:
Trauma (e.g., car accident, sports injury)
Repetitive stress (e.g., heavy lifting, prolonged bending)
Congenital defects
Bone fractures
Tumors

Symptoms:
Back pain, especially with bending or lifting
Stiffness and limited mobility
Numbness, tingling, or weakness in the legs or feet
Loss of bladder or bowel control (in severe cases)

Diagnosis:
Physical exam
X-rays, CT scans, or MRI

Treatment:
Treatment depends on the severity of the condition and the underlying cause. Options include:
Observation and physical therapy
Medications (e.g., pain relievers, muscle relaxants)
Bracing
Surgery (to fuse the affected vertebrae together)

What has Google search returned?