Standing motionless and erect; at attention. Can describe a military posture or someone frozen in place from shock.
兵士たちは号令とともに直立不動の姿勢を取った。
The soldiers assumed the at-attention posture when the command was given.
彼は突然のニュースに驚いて直立不動になった。
He was so shocked by the sudden news that he froze in place.
授業中、先生が入ってくると生徒たちは直立不動になった（冗談めいて）。
When the teacher entered the classroom, the students froze in place (said jokingly).