Google is calling this a "modular base for Android," and says it is the biggest change to the low-level system architecture of the operating system to date.
The company explained in a blog post that feedback from its device-maker partners shows that "updating existing devices to a new version of Android is incredibly time consuming and costly." It doesn't just involve implementing new code and making sure the software works with the hardware: there are several rounds of approval involved.
After Google publishes the open-source code for the latest release, chip manufacturers have to modify the script for their hardware. They then pass the edited code to device makers, who incorporate the new software, making changes as needed. When a final version is ready, device makers have to get approval from carriers before pushing that update out to your phone.
Project Treble cuts down the amount of hardware-accessing code that needs to be written by wrapping all of it in one package that developers can just re-use across each new version of Android. This eliminates the need for chip makers to modify the original open-source code that Google releases and speeds up the approval process.
Since the company is calling this a "modular base," it is likely that more bundles are coming to the base Android code. There aren't many other details on how this system will work just yet, but given Google I/O is happening next week, we expect to hear more very soon.