I would like to remove PlaceHolderPageDescription. But I get error something like PlaceHolderPageDescription couldn't be found. Why sometimes you cann't remove a placeholder from a MasterPage? is it the relationship with Page Layout? What Can I do to remove PlaceHolderPageDescription? I am using SP 2013 and Publish Sites. thanks
Don't remove the PlaceHolders from MasterPage, bad bad practice! As they are used by SharePoint internally to place content accordingly on the pages...
Below URL gives the list of Default Content Place Holders and information that is kept into them [It is for SharePoint 2010, but I hope not much changes are there]:
If you don't want the PlaceHolder, you can either remove the asp:Content control from the pages individually or do as described in the below URL:
I hope this helps