The difference between TrueView In-Stream Ads and Bumpers Ads explained:
TRUEVIEW IN-STREAM ADS:
Non-Skippable Version: Non-skippable in-stream ads are video ads that may appear pre-, mid-, or post-roll while viewing partner content. They can be up to 15-20 seconds long and viewers must watch the ad before they're able to watch the selected video.
More info: https://support.google.com/youtube/answer/188038?hl=enSkippable Version: Minumum 12-seconds videos (less than 3 minutes recommended) charged on CPV or CPM basis.
More info: https://support.google.com/displayspecs/answer/6055025?hl=en
BUMPER ADS:
A 6-second (non-skippable) video that appears before your video on YouTube which is intended for maximum reach and awareness - charged on CPM basis
More info: https://support.google.com/adwords/answer/7159363?hl=en
Here is an example of one I produced:
