I highly recommend it
You're welcome!
It means your video was set as an advertisement before another person's video.
YouTube allows you to set any video as an advertisement, and there's a small chance that it will actually become an ad on other YouTube videos.
That is correct. Because your video IS an ad, it does not have Ads of its own, so you do not get a large monetary gain from this.
The big thing about this is that you can gain new subscribers, views on other videos, likes, and comments. It's more for exposure, which will help you gain more on the videos you monetize in the future.
Go ahead. Hopefully it works for you!