Master pages are meaned to use with Content (Webforms) pages.
So, when you create a master page, You will have to fix a place where the child page (content page) will be incorporated. hence you will put some
ContentPlaceHolder
inside your master page, you can put them in either
head
or
body
part of the page.
every html page can have only single
html,head,body,etc.
tags, so if you are using master page you will put this tgs in master page so child page can not have any of this tage once gain. you can put only
<asp:content />
tags.
when you are creating a Webform (content page), and assinging a master page to it. you have to specify where this form will reside in master page. so you have to select a
ContentPlaceHolder
id.
mark as answer if satisfied with the solution, it motivates :)