Populist-left economics focus on large-scale government spending. The first-order effect is that everybody gets a $2000 check, or a subsidized rail system, or whatever. The second-order effect is inflation and economic instability; you end up paying $10 for a loaf of bread.
Populism is the idea that there is a clear will of the people, or a clear set of policies in their interest. Policy debates aren't divided by left vs. right, or by supply-side Keynesianism. Rather, they're a division between the people and the elites. There isn't really room for a synthesis of multiple competing ideas (at least not until the people have taken over from the elites once again). It's the opposite of pluralism.
Populist economics, according to this author, is defined largely by ideas that are simple enough to sell to people who haven't studied economics enough to conceive of second- and third-order policy effects. Those ideas all fail in the real world, but we have to re-learn that periodically, because of periodic resurgences of populism.
(I believe some of this but not all of it and am really just trying to do my best to articulate what I thought the article was saying).