The "Title of Source" refers to the title of the specific work you are citing - so an article, chapter, or essay title, or a book title if you are using the entire book.
If you are citing something like an encyclopedia article, journal article, essay, or any other piece of information that is part of a larger work, you will need to include the title of that "container." For example, if I am citing a chapter of a book, the title of the chapter would be the "Title of Source," and the title of the entire book would be the "Title of Container."
For more information on formatting Title of Source, see pages 121-134 in the MLA Handbook. For more information on formatting Title of Container, see pages 134-145 in the MLA Handbook.