"pay off" 在此处表示“还清（债务、欠款）”，"all one's debts" 指“某人的全部债务”。因此整个短语的意思是：还清某人的全部债务。
该短语是及物动词短语，通常作句子的谓语，后面需接宾语（即"all one's debts"，其中"one's"需根据主语替换为对应的物主代词，如my/his/her/our/their等）。
After 10 years of frugality, he paid off all his debts last month.
（经过10年的节俭，他上个月还清了所有债务。）
We need to pay off all our debts before we can save for a vacation.
（我们得先还清所有债务，才能存钱度假。）
过去式：She paid off all her debts after getting a promotion.
（她升职后还清了所有债务。）
现在进行时：They are paying off all their debts by working extra hours.
（他们正通过加班还清所有债务。）
将来时：I will pay off all my debts by the end of next year.
（我会在明年年底前还清所有债务。）
All his debts were paid off by his brother when he lost his job.
（他失业时，所有债务都由他哥哥还清了。）
"pay off" 还有“取得成功；得到回报”的意思（此时不涉及债务），需根据语境判断：
His hard work paid off—he got the job.
（他的努力有了回报——他得到了那份工作。）
总结："pay off all one's debts" 是描述“彻底清偿债务”的常用表达，核心是“还清全部欠款”，用法上需结合主语调整物主代词，并根据时态变化动词形式。