。
Swing your arms back and forth.
Literal
Arms [object-を] back-and-forth [locative-に] swing-[soft imperative-なさい].
A physical-education or exercise instruction. 前後 (ぜんご) 'front-and-back, before-and-after' is a compound noun used adverbially. 振る ('to shake, swing, wave') is the standard verb for swinging motions. ~なさい is the soft imperative, typical of exercise instruction or parental directions.