Essentially a copyright claim is someone asserting that you have used their content without their permission, whether that be a video clip, an image, or a piece of audio.
A copyright claim (also known as Content ID claim) is not the same as a copyright strike (aka Copyright takedown).
A copyright claim is automatically generated when an uploaded video matches an asset, and it does not negatively affect your channel or video in any way. However, if you are a monetising channel, the claim may cause advertising revenue to be diverted towards the content owner if their policy is set to “monetise”.
It is possible to have more than one copyright claim on a video, and content ID claims do not create or lead to copyright strikes.