I think directly translated from the Latin it means "compelling law".
The difference between this and customary law is that in customary law you need to have a group of nations accept the treaty/whatever and it needs to be done over a long period, as well it can be altered etc.
The notion of peremptory norms is that there are certain acts/obligations that no state can violate e.g. it's prohibited to be involved in genocide, war crimes etc etc and peremptory norms also applies to individuals (e.g. Milosovic, Nazis etc etc) - not just states.
The Vienna Convention on the Law of Treaties made any treaties between states which would violate a peremptory norm void.
In legal history, it's a relatively new concept and there's still some level of debate going on about it especially people crying "state sovereignty!", but it's not as controvertial as it used to be.