|Their original main signal of quality was backlinks. If someone links to your site, at one time that was normally a signal or vote of approval. But because spammers (or linkbuilders) began creating "unnatural" backlinks to game the system, that signal has lost much of its value. |
So now Google may have started using other signals of quality in addition to backlinks. For example, if visitors tend to spend a lot of time exploring your site, bookmarking pages, printing out pages, returning for repeat visits, etc, then these are other possible signals of quality that the algorithm can use.