Google never want to you leave the web, or better yet, the Google home page. That’s where its advertising model is, and it needs users to see those ads. This is where Instant Apps come in.
Instant Apps let you interact with a native app without installing it. Now, when you’re chatting with your friend (maybe in Allo?), and she wants to show you something from Buzzfeed Video, you can see the video by clicking the url she provides, instead of downloading the entire native app.
Google engineers point out that they expect Instant Apps to only take up under 4 MB of space. The goal is to make user experiences as seamless as possible. Developers will be able to add this functionality to their apps by breaking them down into modules.