A clickable navigation element on a webpage. Clicking on a hyperlink -- commonly simplified to just "link" -- takes you to another webpage or resource (images, documents, PDFs, etc).
Links are often represented as blue and/or underlined text on a webpage.
Links are the "currency" of the Web. Links are an important way for users to discover and browse new websites.
Links are important for SEO. Search engines heavily factor links into their ranking algorithms (#2 most important factor) because they are the online equivalent of a personal referral.