Apple has a similar process in place for OS X called Gatekeeper (which isn't without its flaws), and Windows 10 blocks non-store apps, as well. When you come across an unverified app, you'll get a notification alert of that fact. To proceed, you'll need to type in "continue" in the provided field, which lessens the chance you'll just hit an "ok" button and potentially get exposed to malicious code. As TechCrunch notes, the warning also contains the name of the app and its developer to further help you screen out phishing attempts. Developers can also test out their apps without having to wait for verified status, either, since users can bypass the warning.
"We're committed to fostering a healthy ecosystem for both users and developers," writes Google's Naveen Agarwal and Wesley Chun in a blog post. "These new notices will inform users automatically if they may be at risk, enabling them to make informed decisions to keep their information safe, and will make it easier to test and develop apps for developers."