A commissioner is a member of a commission, or the leader of a governmental body. Commissions are commonly created to solve specific problems or address specific ongoing issues, and are staffed with experts in those areas, these experts would be commissioners.
In 2267, the Federation maintained some form of commission for the purpose of mediating disputes; this body dispatched assistant commissioner Nancy Hedford to Epsilon Canaris III. Her goal was to prevent the outbreak of war; her mission was cut short when she developed Sakuro's Disease and had to be recalled. (TOS: "Metamorphosis")