To be honest, there's not a huge amount to explain. It can cause babies to be IUGR (not grow well or stop growing), it can cause placenta problems, but it's not a given that it'll happen, and most women go on to have completely normal sized babies with normal births.
If you look online, you'll see a load of horror stories of people who say they've had complications, but even more stories of people who are worried about it but never have anything happen.
It's unfortunately a wait and see, they'll usually do growth scans from 28 weeks to check baby is growing properly, you might see your consultant again, but if there's no problems with the babies growth or cord blood flow, you will most likely just stay being midwife led. Has he suggested taking low-dose aspirin?
My first was born 2 years ago and PAPP-A wasn't mentioned, so we don't know if it wasn't a thing they looked at then or if it was normal. I ended up being induced at 36 weeks as he had stopped growing at 32 weeks and the low growth had made my BP increase and there was some debate over pre-eclampsia signs - so nothing to do with low PAPP-A.
Baby 2 is due shortly, my 12 week checks showed a low PAPP-A result, I was already taking low dose aspirin to counter what my new consultant thought had happened in my first pregnancy. This baby is growing ok, he's still small but growing nicely - but coming early as I have other complications this time 🙄 nothing has been said about the low PAPP-A at all, other than initially when it was detected! 🤷🏻♀️
Each pregnancy/placenta is different, so even if you have it once, the chances of it happening again are pretty slim! I've been unlucky with my pregnancies, but it turned out the PAPP-A issue was not the cause of any of it! Try not to worry, if you have questions by all.means speak to you midwife, but try to relax and enjoy the next 6 months or so, it'll probably not mean anything other than seeing baby loads more times at scans :)