--just refers to "small fiber": those nerves that are sensory and/or autonomic and only lightly sheathed, or not at all sheated, in myelin (mostly the latter).

A "samll-fiber" neuropathy--very common for people suffering from vascultiis, autoimmune conditions, or diabetes/impaired glucose tolerance--is simply one in which those are predominantly involved. Since these nerves subsume the sensations of pain and temperature, small-fiber neuropathies typically result in nerve pain, numbness, and temperature disruption sensations.