If you receive a copyright strike, it means that a copyright owner submitted a complete and valid legal takedown request for using their copyright-protected content. This results in YouTube removing your video to comply with copyright law. A few things to note:
- A video can only have one copyright strike at a time
- Deleting a video with a copyright strike does not resolve the issue
Depending on the number of copyright strikes you've received in the past, what happens next will differ - see here. You can resolve a copyright strike by waiting for it to expire, getting a redaction or submitting a counter notification.