Tags are descriptive keywords that help YouTube to understand, rank, and position your videos. They also help people to find your content and broaden your reach. For example, your video could appear in the “Up next” section because of its tag metadata.
YouTube limits tags to roughly 400 to 500 characters including spaces (it’s 500 on Creator Studio Classic). Most creators keep adding terms until they see a notification that reads, “Your tags exceed the allowed character count.” When you exceed the limit, you won’t be able to save your edits until you reduce your character count.