a short remark or message added to the bottom of a letter after you have signed your name, usually introduced by the abbreviation PS

There was the usual romantic postscript at the end of his letter - PS I love you. 他的信末又是一贯浪漫的附笔——附： 我爱你。

any written or spoken addition to something already finished

As a postscript to that story I told you last week, it turned out that the woman was his sister-in-law. 对上周我给你讲的那件事补充一句，那个女人原来是他妻子的妹妹。