A topic statement* is the central idea of your paper or project. Writing a topic statement and developing your topic is an important step in the research process. Without a good topic statement you will not have appropriate keywords, or search terms, in order to look for and find resources like books and scholarly journal articles. You must define and KNOW what you are looking for before you begin to look!
*The topic statement is also referred to as the "thesis" or the "research question" depending on academic discipline and other factors.