Populism is basically a political ideology that holds that the “common people” are in some way superior to the elites and that they should exercise more control over the government and its policies. This ideology generally holds that elite ideals and attitudes are harmful to the common people and are less admirable than those of the common people.
Populism first arose in the US in the late 1800s and early 1900s. At that time, it was a response to industrialization and modernization. The populists felt, for example, that the elites were using the government for their own economic benefit and to harm the common people. The common people wanted things like regulations on railroad rates so that the rich railroad owners would no longer be able to charge exorbitant prices to small farmers who needed the railroads to transport crops.
Today, there is still an economic aspect to populism. Populists tend to feel, for example, that big companies are sending jobs overseas and/or are getting government bailouts without doing anything to help the common people. They feel that this should stop. Populists today also typically oppose what they see as elite values. They tend to believe that the elites’ values (on things like patriotism and religion, for example) are less authentically American than those of the common people.
Populism, then, is the political ideology that believes that the common people should have a greater voice in the government because their values are superior to those of the elites.