A companion ad is a traditional graphic or Flash ad which is paired with an in-stream video ad (pre-roll, mid-roll,..) or a video overlay from the same advertiser.
The companion ad is more often a traditional display ad (often a rectangle) which is displayed adjacent to the video player for the duration of the main video content.
The companion ad may also be a skin that wraps the video player or more rarely a background skin for the page.
The goal of the companion ad product is to reinforce the advertising message by sustaining visibility of the advertiser throughout the video content experience.
The companion ad is usually sold as a package with the video ad unit by video platforms like YouTube.
An example of companion ad: