What does mild facet fluid in l3-4 and l4-5 region mean?

Hard to say: May mean nothing. May mean early changes of arthritis.

Nothing much: Facets are the left and right joints of the back of the spine present at each level such as L3-4 and L4-5. They contain fluid because as joints they need lubricants. Sometimes MRI can show this fluid. If there is only mild amount seen, it probably means very little. When there is a lot, we spine surgeons feel that there is possibly too much movement, perhaps inflammation, and possibly cause pain.