If you set this option to true, all TinyMCE will convert all font elements to span elements and generate span elements instead of font elements. This option should be used inorder to get more W3C compatible code, since font elements are deprecated. How sizes gets converted can be controlled by the font_size_classes and font_size_style_values options.
More information about this subject can be found in this article: What's wrong with FONT FACE
Example of usage of the convert_fonts_to_spans option: