Relevance is important because you are expected to support your ideas with pertinent information.
You and your instructor are the best people to determine if a source is appropriate for your research. Remember that all sources you use should contain well-supported arguments and valid research.
A source detailing Einstein's marriage and family life would not be germane to his theories in physics.
Does the information answer your research question?
Does the information meet the stated requirements of the assignment?
Is the information too technical or too simplified for you to use?
Does the source add something new to your knowledge of your topic?
Look at the preface of author's notes for information about the book's purpose. For web sites look at About Us/Mission links, content, and advertising.
Judge whether the source is scholarly or popular. Make sure that you are using the appropriate type of source for your assignment.
Look for a bibliography or works cited list. Use the bibliography at the end of the source to find other related works.
Look at the abstract, table of contents, introduction, and index to help determine its relevance.