What are Copyright Strikes on YouTube?
A copyright strike is when a copyright owner sends a legal request to youtube to take down a video. For example, if you post a video of yourself while Justin Bieber music is playing in the video without you applying the copyright common licence, then your video will get the copyright strike. Thats why you see claims such as “I do not own the music that is playing in this video, I will respectfully take down this video if i need to” something like that, to prevent copyright issues.