Salus populi suprema lex: the quote is from Cicero, and means The health (welfare, good, salvation, felicity) of the people should be the supreme law2. There's actually an extra "esto" you need to shove in to make good latin, but it kinda reads nicer without, or at least I presume that's what the people who made my monument1 thought. It is also the form that the Inns of Court use, and their latin is likely better than mine.
Like quite a lot of things, it sounds good, certainly good enough to write on a monument or use as your great seal. Nowadays, if you search for the phrase on Google your results fill up with fuckwitted colonials who think that taking the word "health" literally is amusing. Idiots. Anyway, where was I? Oh, yes...
It has a regrettably collectivist, as opposed to individualist, slant. The welfare of the entire populace demands that your individual rights should be overridden? Then over they go... You see the problem I hope. Or not even individual rights: taken literally, it offers the sovereign the ability to cast any specific law aside, in favour of the "supreme" law: in both early modern and classical contexts, salus populi was often used to justify the 'arbitrary' power of the sovereign, in the words of one random source I found on the internet. From context, and as this argues, the maxim is only supposed to apply in times of emergency; but alas people can be rather slippery about what counts as an emergency.
Will that do for now? Not one of my best, I know.
* Primitive communism by Manvir Singh via Econlib
1. On Berkhamstead common.
2. Partial context, from an archive of Loeb: In the field they shall hold the supreme military power; they shall be subject to no one; the safety of the people shall be their highest law.