Your business partner emails you a .docx file. You open it, intending to make a few changes, and then realize that you can't make changes to the document because Word opened it in Protected View, where you can see but not edit the content. As usual, Microsoft is protecting you from yourself, on the off chance that you've opened a Word document with a macro virus in it. There's an Enable Editing button at the top of the screen that will let you edit, but you shouldn't even have to click that.
To open all Word 2013 documents in editing mode from now on, select Options from the File menu, select Trust Center and click Trust Center Settings. Select Protected View, uncheck all three options and click OK.