Thanks for the tag, ads will appear on videos if your channel is part of the YouTube Partner Program. You can choose how to turn off the ads: https://yt.be/help/MWH8 & set your monetization default here: https://yt.be/help/R3GV
If you're not yet part of YPP & you're seeing ads on your video, this might be due to a copyright claim. You can check if your video is listed under the copyright notices section: https://www.youtube.com/my_videos_copyright …