A copyright strike means that a content owner has submitted a legal takedown request in order to protect their asset(s).
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.