Originally a military term — relieving sentries at a guard post. Now also used for any shift change at a fixed post: factory floor, security desk, hospital ward. The related noun 岗位 (gǎngwèi) means 'post / position' more broadly.
memory
换 'to swap' + 岗 'post / sentry station' (岗 originally meant 'mountain ridge' — a high lookout point — and that's why it became the word for a guard post).
