About five months ago, Google introduced additional index status reports that showed a lot more information about how Google indexed your site.
Recently, a lot of webmasters have been asking about the "not selected" number. Technically, Google explains it as "URLs from your site that redirect to other pages or URLs whose contents are substantially similar to other pages."
John Mueller of Google explained it a bit more in depth in this Google Webmaster Help thread saying:
The number of "not selected" URLs is based on URLs that are either substantially similar or redirecting -- if you have changed your site's URL structure and have redirected those URLs, then that would be a good explanation for that. That curve would also be fine and not a signal of a problem.
Now, what happens when your not selected number is well above your other numbers, such as index count? Should you be worried? I might be. It may show signs of some structural issues with the site.
Here is one chart of someone who has such an issue:
Now, to me, this is a bit scary. Can a site like this do well in Google? It is possible but look at all that lost potential.