What is a YouTube Masthead?
The Masthead is a customizable 970x250 pixel ad unit that runs on the desktop YouTube homepage for a 24 hour time slot. This unit also has an expandable option that can expand on user click to 970x500 pixels. YouTube highly recommends the use of videos in custom Mastheads to provide the most compelling and native ad experience.
See the Custom Masthead Guide in the Display Specs Help Center for more information.
This guide explains everything you need to know about the process of launching a YouTube Masthead.