What most of us want when we say we want to live in a democracy is not a true (absolute) democracy, but a limited democracy. We want a democracy within limits. In particular, we don't want 51% to be able to vote to take away the rights (or the lives) of the 49%. In an absolute democracy, they could; in a limited one they cannot.
Populism is, essentially, turning the democracy into mob rule. If you don't have limits, then the mob rules. If you do have limits, populism tries to erode them so that the people can do what they want.
Note also how populism is often associated with authoritarianism. Someone who wants to be the authoritarian persuades the people that he will get them what they want/need, if they just remove those pesky limits to his power. He encourages them to tear down the limits to government power in the name of the people getting what they want - that is, in the name of democracy. But when the people want to reign in the authoritarian's power, they find it's not so easy, because they gave the new power to the authoritarian...