What does 'mild' osteoarthritis of the spine really mean?

Nothing: It means you're 38! like wrinkles of skin or gray hair, your spine also normally ages. These findings are reported as "arthritis" or degenerative disk disease or spondylosis. None of these implies any pain or need for further treatment nor restriction if activities.

What it says: There is no other way to explain it other than to say that x rays show mild oa rather than moderate or severe oa. It is a matter of degree.