The Source page shows the raw XML source for the plug-in manifest file. You can do all of your editing in this page, but you will not be able to validate the page as you go. The form editors can provide much more assistance in editing the file correctly.
If a syntax error is introduced, PDE will lock out the form pages until the error is fixed. If you try to switch to any of the non-source pages, you will see the following dialog:
If you close the editor with errors in it and open it again, the editor will open into the Source page and other pages will not be accessible until the source is fixed.