Classism is discriminating against people because of how much money they have, their level of education, or the kind of job they do. Assuming someone is poor because they're too lazy to work is an example of classism.
Classism, like racism and sexism, shows up in an individual's prejudice against poor or uneducated people but also affects government policies, company hiring, college admissions, and more. The fact that wealthy citizens have more power to influence politics than poorer people is a form of institutional classism. And if you think people who own mansions are better than those living in tiny apartments, or that a lawyer is smarter than an electrician or plumber, that's also the result of classism.