A letter ends, then a thought arrives late and gets added as a PS. If a second thought arrives after that, it goes in as a PPS. The letters stand for post postscript, so each extra P marks one more step past the ending. You write it on its own line, usually with a colon, and then the note. The source marks this use as American English. It carries a light, chatty feeling, because it shows the writer talking on after they meant to stop, so it suits cards, notes and personal email far more than an official letter.

This PPS is a person, not a note. The source describes the job as helping a minister, and the person doing it is an elected MP rather than office staff. It is a junior post, so you meet it in sentences about someone taking the role or leaving it: she was appointed PPS to the minister, he stepped down as PPS. The word almost always comes with to plus the minister or the department. It belongs to British political news, so a learner is most likely to meet it while reading about the UK, and it needs no translation into other countries' systems.