There’s thus really good video on populism by Ryan Chapman (on YouTube) that explains in really well, but in a nutshell, populism is the idea that this group/leader represents the people and is fighting against “the establishment”. It is the converse of pluralism, which is the acceptance of many different ideas. Populism is not inherently dangerous but it is often associated with authoritarianism (and rightly so) due to its anti-pluralistic nature.

Ryan explains it a lot better than I can do in an internet comment. I find he has a pretty factual and balanced take on the topics he discusses.