I think another name for this is Bertolotti's syndrome which might be easier for you to look up. As I understand it, in my non-medically trained mind..., it is a situation that is congenital, and doesn't usually start to become symptomatic until the twenties or thirties.
Basically there is an "irregular" joint that forms where the last lurabar vertebra joins with the sacrum, a sacral vertebra or the ilium. I believe it is almost like a natural fusion of one to the other. Just as in a surgical fusion, the immobility of this "joint" puts additional stress on the next upper vertebra (L4) and the adjoining spinal nerve roots...This can result in degeneration and it is this that causes the lower back and/or sciatic pain.
This is a layman's interpretation of anomalous articulation....I'm sure you'll be consulting with your doctor to find out what your MRI shows and what it means in terms of your own pain.