It is a new feature which was presented in SharePoint 2013 which allows to force search service to re-crawl whole list or library regardless if items where changed or not. Items will be picked up by full crawl any way but if will be marked for re-crawl they will be crawled also during incremental crawl.
This feature is very useful when new managed property is configured or site column is modified so as a result search schema needs to be updated and list owner who initially has not access to central admin to fire full crawl which will be very expensive step for large deployment and will affect a lot of content.
So this cool feature can be found in list (I will not mention library anymore) settings under Advanced Settings link.
Scroll a little Advanced Settings page and pay attention to Search block where you can enable the list to appear in search results. This option should be enabled otherwise Reindex Document feature will not have any sense.
Only then you can press Reindex Document Library or Reindex List button
In popup dialog just confirm that you are sure and want to re-index list.
Now your list will be picked up by next continuous or incremental crawl. Make a note that you do not need to scroll down on Advanced Settings page and press OK, list already marked as dirty.
I am not sure if someone will need to make the same using PowerShell or OM, but anyway drop a line in comment then we can discuss it.