Reduction of graphene oxide means deoxygenation of graphene oxide(GO) via chemical or physical  processes.Graphene oxide is a product of oxidation and exfoliation  of graphite(G).Graphite has high  C/O ratio of 6/1 to 8/1  and low surface area.

  Graphite is multilayer conductive material while graphene oxide is semi conductive material  which has C/O ratio of 2/1 .The main reason behind oxidation/exfoliation is to produce mono- to few-layer  graphene oxide and increasing  surface area. On  oxidation process ,graphite oxide lose most of conductive properties of graphite due to disordering of aromatic system. To restoring some or most  conductive properties of graphite , reduction process must be performed .On reduction, reduced graphene oxide(rGO) restores aromatic π → π system and hence rGO may has high C/O ratio of 4/1 to 6/1 and also restores most  conductive properties of graphite.

Please for the answer check the following paper:

Tailoring the Oxygen Content of Graphite and Reduced Graphene Oxide for Specific Applications

Naoki Morimoto, Takuya Kubo, Yuta Nishina

Scientific Reports 6, Article number: 21715 (2016)

doi:10.1038/srep21715