Mononuclear cells are types of white blood cells and the term "rare" would mean not a whole lot of them. The "normal" number of those type of cells in the cerebrospinal fluid is anywhere from none to very low (rare if you will). If they were elevated, it could indicate a number of different things . . . for example . . . a viral infection of the nervous system (e.g. meningitis) or multiple sclerosis.
It appears as if your husband's levels were normal, though.
I hope that helps. If it doesn't, please let me know. Take care.