。
Extend your arms straight.
Literal
Arms [object-を] straight extend-[soft imperative-なさい].
An exercise or PE instruction. 伸ばす ('to extend, stretch out') is the standard verb for stretching a body part or a line/object. まっすぐ is the adverb 'straight, directly'. ~なさい is the soft imperative typical of exercise instruction or parental directions.