What?
A copyright strike means that a content owner has submitted a legal takedown request in order to protect their asset(s).
Why?
YouTube is legally obliged to obey this request, and will remove your video from the platform. The first time you receive a copyright strike, you will need to complete YouTube’s “Copyright School” programme. If you get 3 strikes, your account, channel(s) and video(s) will be removed from YouTube. If you are a member of the YouTube Partner Programme with 3 copyright strikes, you will have a further 7 days to resolve your copyright strike.
More information can be found here.
It is not possible for a single video to have more than one copyright strike.