The basic idea behind individualism is that the individual person and his or her interests are a very important (or perhaps the most important) thing, the thing that society should be most concerned with protecting. This is as opposed to valuing and protecting the community as a whole, for example, or any other institutions larger than the individual.
You ask about this specifically in the context of US History II. Depending on what is covered in your version of this class, you might be referring to Pres. Hoover's idea of "rugged individualism." This is the idea that people should take care of themselves rather than depending on the government to help them. This idea is one aspect or facet of the more general idea of individualism.
Overall, individualism is a philosophy that stresses the importance of the individual and his or her interests, rights, and freedom. It is one of the bases of American society and values.