Left s1 nerve root within the lateral recess against the l5/s1 facet joint. what does this mean?

Depends: It is not really a full description of what sounds like a radiologists reading of an MRI report. The s1nerve root exits out of the spine through the neural foramen next to the l5-s1 facet. I suspect the radiologist is desribing an area of stenosis in that area. A simple way to describe that finding is to say you have a pinched nerve in your back. Thank you for your question.

Facet nerve anatomy : This could be a normal position for the nerve to be. As the nerve exits the spinal canal, it courses under the facet joint. If leg pain is present (sciatica), the facet joint may be degenerated and pressing on the nerve. At your age, facet hypertrophy or enlargement is not common. Enlargement is usually in older individuals.

Means: that the facet joint at L5/S1 is in contact with the nerve root suggesting maybe the facet joint has some growth or arthropathy (arthritis/bone spurs) that could irritate the nerve and cause pain. Without more info hard to say anything else.