Google, as a search engine, has the goal of serving searchers with exactly what they are looking for.
So quality content is that which answers the searchers question, or meets the searchers need.
Long informative articles with images, videos and good readability tend to rank higher on the search engines because it engages the searcher more.
Google also ranks your site based on how searchers react to your site. If they click through to your site and the content isn't any good then they will leave the site and keep searching for what they need.
This is also an indication for google to rank you lower, so high quality content that creates user engagement rank higher on google.