"The links shown below some of Google's search results, called sitelinks, are meant to help users navigate your site. Our systems analyze the link structure of your site to find shortcuts that will save users time and allow them to quickly find the information they're looking for," explains Google.
Here's the new interface:
... and here's what happens when you expand a sitelink:
This is the regular mobile UI:
Google mentioned a similar feature for the desktop site last year: "this improvement digs deeper into megasitelinks by showing sub-sitelinks instead of the normal snippet." I haven't noticed this feature and it doesn't seem to be available in the desktop interface.