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Zhaban, a northerner
A northerner was a type of Romulan. Ridged foreheads allowed one to be recognized as a northerner.
Northerners were seen throughout the Romulan capital city, including the Krocton Segment. (TNG: "Unification I", "Unification II")
By 2379, the entire Romulan Senate was made up of northerners. (Star Trek Nemesis)
In 2399, while interrogating a Romulan operative at Château Picard, Zhaban told Laris that they would never get anything out of him. Laris angrily responded by saying that it was because he was "a stubborn northerner, like you!", while tapping their captive in the forehead. (PIC: "The End is the Beginning")
Background information
Northerners offer a solution to the longstanding situation of Romulans having been depicted both as having ridged foreheads, and smooth ones, depending on the series. Star Trek: The Original Series and its sequel films presented ridge-less Romulans, while ridged make-up became the norm in The Next Generation, Deep Space Nine, Voyager, and Enterprise, as well as Star Trek Nemesis. The 2009 movie again showed smooth-headed Romulans. However, Spock's ability to go unnoticed on Romulus in "Unification I" and "Unification II" implied that smooth-headed Romulans were still around, unseen, and some Romulans in The Original Series wore helmets, obscuring their foreheads.
Star Trek: Picard became the first series to commit to showing a mix of both variants. That series also offered an explanation, in "The End is the Beginning", where Laris drew attention to a Romulan's ridged forehead while visually identifying him as a northerner. According to the audio narration in the Amazon Prime release of the episode, Laris specifically flicks the captive Romulan's forehead ridges when she calls him a northerner.
With no definite in-universe explanation available for many years, various non-canon sources have weighted in on the variety. StarTrek.com suggested that the ridged majority of Romulans were a different race that evolved on Vulcan simultaneously with them. [1](X)
In an interview, Neville Page, the head art designer for 2009's Star Trek had a different explanation. During his work on the movie, he first created a back-story to justify the change their faces had undergone, explaining that as a result of their grief, anger, and general bad-ass persona, the Romulans chose to cut and scar themselves, leaving behind such significant keloids on their foreheads that it eventually wended its way into the gene pool over many years, eventually becoming a natural characteristic of all Romulans and thus creating the distinct difference between them and their Vulcan cousins. [2]