Link rot refers to the fact that as a website exists for some time, links on it will inevitably stop working. Either because internal pages have been moved but not redirected, or because external websites you're linking to changed their URL or went out of business.
Link rot hurts the user experience, because no one likes to click on a link that doesn't work. Additionally, it can hurt your SEO, because broken links no longer work.
Finding and broken links is part of your work as an SEO.Back