What does lumbosacral junction is assumed l5 s1 mean?

Your spinal cord is made up of 5 different areas, based on how many vertebrae (pieces of spine) make it up.

You have:

7 CERVICAL

12 THORACIC

5 LUMBAR

5 SACRAL

5 COCCYGEAL

Therefore the lumbosacral junction (where lumbar meets sacral) is assumed to be at L5 (the 5th and last lumbar vertebrae) and S1 (the 1st sacral vertebrae).