アクが出なくなったら清潔な瓶にシロップを戻し冷蔵庫で保存します。

。

Once the scum stops forming, return the syrup to a clean jar and store it in the fridge.

Literal

Scum [subject-が] stops-coming-out-[when-たら], clean jar [locative-に] syrup [object-を] return, refrigerator [in-で] preserve.

A cooking instruction. アク in cooking means the foam/scum/impurities that rise when boiling or simmering food — a standard term in any recipe. The verb 戻す (to return) is used here for 'put back'. Note the lack of explicit conjunctions between clauses: ~て-form on 戻し links two actions, a very common written/spoken pattern for listing sequential steps.