Relative URLs are often used by web developers as shorthand to code internal links on a website. They do not contain the full URL, but still link to the destination page. Absolute URLs contain the full URL string when linking internally to another page.
Why does it matter?
Relative URLs are SLIGHTLY better for page loading times and easier for developers when coding HTML. Absolute URLs are better for SEO as they contain the full URL string, better optimized for search engine crawling.