@gendal covered it. It’s the inclusion of nitrogen into the gas mix put into the beer in addition to carbon dioxide. Here it’s called, “beer gas,” but I don’t know if that term is popular everywhere. The nitrogen percentage varies, but I’ve seen something like 65-75% nitrogen is normal. Within beer ingredients, it roughly highlights malts, but mutes hops. So like @moss_icon mentioned, IPA’s on nitro are a real mixed bag. But darker beers, with more of a malt mix, seem to highlight when on nitro. It surprisingly also makes the underlying beer taste a tad flatter, because nitrogen doesn’t absorb into the water as much as carbon dioxide does.
There are two things important in a nitro, both the gas used in the beer, but also the spout they are poured out of. That spout is usually special, it has a small plate that restricts the flow into the glass. This causes the nitrogen to agitate going into the glass and it why when you get a Guinness on tap it seems to cascade in the glass as it bubbles. You can get nitro beers in a can and bottle, they aren’t 100% the same, but it’s close. You usually don’t get that cascading pour effect though.
All of this knowledge was passed to me via by homebrewing buddy, the guru of beer, but who passes these tidbits to me during drinking sessions, like an inebriated Obi-Wan Kenobi. “Only a Budweiser drinker deals in absolutes.”