Fuku (副) is set as a suffix in front of the title and means deputy, representative, supporting or vice.
The word Shidōin (指導員) descends from Shido (suru) 指導 (する).
It consists of yubi 指 which means finger or sasu 指す which is the verb for showing.
[1] Different budo arts and dojos have several requirements for the usage of this title, but in general it corresponds to 2nd or 3rd Dan.
[2] This article related to martial arts terminology is a stub.