This sounds like a diagnosis someone discovered on a medical
report.
Mild is a rather obvious term, meaning only slight and perhaps
the early phase of a problem.
Bilateral means on both sides or opposite sides of a part of the
body, usually meaning on the right and left side of the body.
Facet refers to the joint or articulation where two parts of the
spine connect at the back part of the spinal column. A facet joint
is a small structure that acts like a hinge or point of contact
between two connecting spinal bones (vertebrae) to offer support
while still allowing movement in the spine.
Osteoarthropathy is a term meaning early osteoarthritis, in
which it is suspected that early joint cartilage softening and
breakdown has occurred, with possible slight or mild bone changes
that are typical of Arthritis.
Overall, the term means a doctor believes that someone has early
osteoarthritis in the facet joints in an unidentified part of the
spine, probably causing mild to moderate and occasional spinal pain
and reduced movement when aggravated by excess movement or
standing.
