Translation:People are clever.
ㅇㄹ -> ㅇㄴ
a very straightforward and simple rule
영리 -> [영니]
승리 -> [승리] (victory)
정류장 -> [정뉴장] (bus stop)
릉라도 -> [릉나도] (Rungna-do, an island in Korea)
similar rules with ㄹ:
ㅂㄹ -> ㅁㄴ
십리 -> [심니] (ten Ri; Ri is a distance measure unit)
ㅁㄹ -> ㅁㄴ
침략 -> [침냑] (invasion)
ㄱㄹ -> ㅇㄴ
독립 -> [동닙] (independence)
Whoops, it should be written 《승리 -> [승니]》in the 2nd example. My bad.
As a bit of trivia, in North Korea it is allowed, and even encouraged, to pronunce ㄱㄹ as [ㅇㄹ], ㅂㄹ as [ㅁㄹ], and ignore the ㅇㄹ and ㅁㄹ rules whatsoever. In effect, this eliminates the need to change the pronunciation of ㄹ under any circumstances. It is rare to hear anyone speak this way, though.
They're talking about people in general, which we pluralise in English.
You could translate it to "A person is smart", but that sounds awkward in English.
If it was pluralised, it might probably be more like saying "Groups of people are smart" (like a sum is greater than the whole)
Also: "A person is smart. People are dumb panicky animals and you know it."