Short (up to 6 seconds) video ads with brief messages that are displayed on YouTube. Google Bumper Ads are great to reach out to people who are in a hurry, as well as to drive upper-funnel goals (e.g. awareness). Such ads are unskippable which ensures a nearly 100% ad completion rate.
This type of ads target CPM bidding — advertisers pay for every thousand ad displays. Even though these ads can be used independently, Google recommends running them with TrueView ad formats.