Title of Source
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.
Title of Container
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 different source titles, see pages 25-29 in the MLA Handbook. For more information on formatting different container titles, see pages 30-36 in the MLA Handbook.