Microsoft released the first major update for Windows 8.1, creatively named Windows 8.1 Update, back in April as part of its regular "patch Tuesday" cycle. The update improved how the Modern UI interface handles mouse and keyboard input, and it also made some minor under-the-hood improvements.
What you may not know is that Microsoft now requires the update to be installed in order for Windows 8.1 to receive any future updates—including critical security updates like the ones issued in the most recent patch Tuesday. June 10 marked the deadline for customers to get fully up to date, according to this post on the official Windows Experience Blog.
If you have Windows Update set to download and install updates automatically, you should be set. If, for some reason, you don't, Microsoft has put up a page to help users check if they have the Windows 8.1 Update installed. The easiest way to check is to look at the Start screen. If there's a Search button next to your name, you have the update installed.
Given the critical nature of patch Tuesday security updates, we wouldn't recommend skipping this one.