What the user sees, what the crawler sees
In recent years, more and more of the web has become populated with AJAX-based applications, replacing static HTML pages. This is a great development for users because it makes applications much faster and richer. But making your application more responsive has come at a huge cost: crawlers are not able to see any content that is created dynamically. As a result, the most modern applications are also the ones that are often the least searchable.