Because of the fog, visibility was only 100 meters.
Literal
Fog [with-で] visibility [subject-が] 100 meters [only-しか] did-not-work.
視界がきく = 'visibility works / one can see' — a set phrase where 効く ('work, be effective') is the verb. しか...ない emphasizes the narrowness: 'worked only up to 100m.'