Question
I can not understand my MRI and would like an explination of findings.  Please help me understand.

There is a transitional vertebra at the L-S.  At L4/L5 there is 6.7 mm broad based disc protrusion and posterolisthesis that effaces the thecal sac.  This measure 6.2 mm in flexion and 6.5 mm in extension.  This produces right neuroforaminal narrowing causing effacement of the right L-4 exiting nerve root.

At L3/L4 there is a 2.5 mm broad based disc protrusion that effaces the thecal sac.  This measure 2.1 mm in tlexion and 2.9 mm in extension.

There is facet joints and ligamenta flava hypertrophy from L3/L4 through L5/S1.

Thank you in advance for your time and help.  

Answer
You are not supposed to understand what a doctor writes, they should be explaining your results to you.  I question why your doctor has not explained it, and if they have, why they didn't explain it more clearly.  
A transitional vertebrae means there is either an extra vertebrae or a connection between the last lumbar vertebrae and the sacrum.  All the places where there is disc protrusion with effacement on the thecal sac means you have a disc bulge that is touching the spinal cord, I don't know your symptoms but that means you could be having or will soon have some tingling or pain to your extremities (legs).  The neuroforaminal narrowing means that the "hole" where the nerve exits the spine to its destination is closing, usually due to repeated trauma from bad mechanics or body alignment.  Hypertrophy means to grow, so the facet joint hypertrophy and ligamenta flava hypertrophy are growing, which can also irritate the nerves.  
I'm curious on what treatments have been suggested, only because I know surgery is very invasive, costly, and sometime dangerous.  I am a chiropractor that treat these same symptoms on numerous people, without surgery or drugs.  I am part of a special group of chiropractors that use a low force technique to assist the body to realign the vertebrae and help to relocate the discs.  If you are interested in seeking treatment, go to www.sorsi.com to find a doctor near you.  If you see a chiropractor already, or have a chiropractic friend, talk to them.  If you are in Minnesota near the Twin Cities, or if you any other question or concerns about any future xrays or treatments that are recommended, please email me.

Dr. Starbuck