It's basically a catch-all term that applies to wide variety of political figures from William Jennings Bryan to Juan Peron, Adolf Hitler, Bernie Sanders, and Donald Trump.
I think it's supposed to mean any ideology that attacks the establishment and appeals to lower class people. The people it appeals to could be farmers, miners, the urban poor, union workers, etc. Of course, every populist movement attracts wealthy people and business interests (Silver mining companies for WJB, celebrities who supported Sanders, and the various business interests and media personalities that supported Trump). It has more to do with a candidate's style than their policies.
