What does a thesis sentence mean in language arts?

In high school and college, a "thesis statement" (as it is usually called) is an important part of any long essay-- such as a term paper. It refers to what your paper will be about, and it explains what the paper will try to prove or demonstrate. It should usually be in the first paragraph, as part of introducing your topic. It is important because if you have not chosen what your paper will try to prove, you will have a hard time finding appropriate research or making the right points to defend your thesis.