According to the Polygon article "Beskar, signets, and why armor is everything in The Mandalorian":
Along with the passing reference to Beskar, the Armorer also asks the Mandalorian if his signet has been revealed. He says not yet, so she makes him a plain, pauldron. We don’t know exactly what this might mean; it seems possible that this refers to a personalized marking that Mandalorians may receive at some point over time, perhaps after a certain number of missions, or at a certain point in their life.
We know that personalized armor is very important to Mandalorians, so a valued signet would be make sense. In Legends, the Mandalorians would color their armor with either specific missions that were important to them — for instance sand-gold armor represented a quest for vengeance — or simply colors they liked. While this isn’t explicitly canon anymore, Sabine does exactly this in Star Wars: Rebels when she decorates her Mandalorian armor herself.