As Brandon Seymour points out in his very clear and non-scary explanation of this Google mobile-friendly April 21, 2015 update, Google could change its position on including tables in mobile-friendliness, since tablets usually do not allow for hovering with a mouse, which usually is done on a desktop. Tablets require tapping, and not all desktop machines use tapping, but desktops do still use a mouse or trackpad for navigating and moving through web pages and computer programs. Right now, the mobile-friendly update applies to phones, as explained here.
According to the U.S. Mobile App Report 2014 from comScore, 60% of all Internet usage happens on a smartphone or tablet. Also according to the report, “The fuel driving mobile’s relentless growth is primarily app usage, which alone makes up a majority of total digital media engagement at 52%.” According to Seymour, Google has the largest mobile app store, and is starting to include content from apps in its search results by people using their smartphones. This means that when someone searches for something, they may get links to pages found within an app (aka "deep links"), not just a website. This would be by Bit.ly, the URL shortening tool, is expanding its services to include "deep links".
Keep tabs (no pun intended) on how your website looks and feels in a tablet design. Google insists via Twitter that the change is for mobile only right now, but the very nature of a tablet is to let a person be not tethered to a desktop computer. The tapping vs hovering function is also a big difference in how a person uses your website. For now, make sure nothing is hover-only in your website design and can be tapped.