Sometimes people forget to update their computers when a new software patch is released. Sometimes they don't realize they should. And sometimes they just don't want to update.
If you use Internet Explorer 11 on Windows 7 and you haven't installed Microsoft's April security update to Explorer, then you won't be able to install the much larger June IE update, and you won't receive any further updates to IE 11 — including important security patches — until you do.
Furthermore, if you're a Windows 7 user and you don't have the April Windows 7 IE 11 patch installed, not only will future Internet Explorer patches not install, but they won't even appear in your Windows Update queue of recommend updates.
Microsoft had previously warned Windows 8.1 users that they had to update to a later version of Windows 8.1 in order to receive any security patches beyond June 10. Now, according to a technically dense Microsoft support document, it seems Windows 7 users are also being required to update something now to get more updates later. In this case, however, ignoring the April update only affects Internet Explorer 11.
This requirement immediately affects Windows 7 users who depend on Windows Update to keep their computers patched — mostly consumers and small and medium-size businesses. Enterprise users, who use different updating software, have until August 12 to install the April IE patches.
The April IE update contained no major changes to Microsoft's products, so if you've been regularly downloading updates, you should be fine installing it. If, for whatever reason, you don't want to, you can always replace Internet Explorer 11 with a different browser.