Your Google account is much more than just the window to your Gmail, the Play Store or your Android phone. In fact, many of us use our Google account to authenticate ourselves and sign-in across all manner of apps on all kinds of different platforms. In modern desktop operating systems such as Windows 10 and MacOS, users can sign in with their Google Accounts to get access to their Gmail, their Contacts and other data, and it's done using the same email address and password you'd use for everything else. This isn't changing, but what Google is changing, is the way that developers can allow users to sign in with their Google accounts.
Specifically, Google is saying goodbye to OAuth instances using third-party browsers. This means that, on platforms that support it, Google will no longer allow apps or operating systems to sign someone in to their Google Account without using that platform's native browser. So, for us Android users that would be Chrome, and on iOS or MacOS it'd be Safari. This change will come into affect starting October 20th 2016, and there's a good reason that Google is making such a change; security. Using a platform's native browser is a near-certain way to use a more up-to-date and therefore more secure browser to sign in. This change will restrict users from logging in to certain apps or platforms after October 20th that don't use a native browser.
For users, the only thing that's going to change is which window they input their information in, but for developers, there's a little more work to do to make sure that their apps allow users to sign in with Google properly. In the same post that Google outlines the forthcoming change, they give developers a helping hand, too. iOS and MacOS developers will be fine, as Safari is already supported, and Windows developers are given plenty of examples by Google themselves. Those developers that need to make some changes can find out more at the source link below, while users will just see a change when they sign into their apps later in the year.