What is a claim?
Broadly speaking, there are two sources for videos:
Partner-uploaded content is videos that you upload to a YouTube channel that you own. You claim the video as you upload it, and apply a usage policy to it. Claiming partner-uploaded content enables monetization options, Content ID matching, and reporting.
User-generated content (also known as UGC) is videos that other people upload to their channels. When the video contains content that you own (as determined by a Content ID match or a manual claim), you claim the video and apply a match policy to it. The match policy determines where and how the video is available on YouTube.