What is a YouTube masthead?
The YouTube Desktop Custom Masthead is a 970x250 pixel in-page unit running the full width of the YouTube homepage below the navigation bar for 24 hours. There is also an expandable version that expands on the user click to 970x500 pixels. See the custom masthead guide in the YouTube Help Center for more information.
This guide explains everything you need to know about the process of launching a YouTube masthead.