Google is reportedly working on a new operating system, according to Android Police. It's not going to replace Android or Chrome OS, but it could serve as an additional choice for an unknown range of products.
The OS, currently called Fuchsia, also comes with another trick: unlike Android and Chrome OS, it's not based on Linux.
Google isn't spilling much information itself. The description on Fuchsia's GitHub page is fairly unremarkable: "Pink + Purple == Fuchsia (a new Operating System)." That's it.
According to Android Police, the operating system's Magenta kernel allows it to work with full-fledged desktop and laptop PCs, as well as various wearable devices. Fuchsia can run on 32-bit and 64-bit ARM CPUs, as well as 64-bit Intel CPUs (and, eventually, Raspberry Pi 3). Looking forward, it's certainly possible that Fuchsia could replace both Chrome OS and Android, but it's a bit too early to call it just yet.
As for why Google is working on a new OS, the current speculation suggests that the company is looking to deploy something that's a bit more lightweight for devices that don't have a full Linux load. Theoretically, Fuchsia would make more sense when used within lesser devices, like IoT products, Google's OnHub router, or a car dashboard unit.
It's also possible that Google's interest in a new operating system is just that—interest, not an indication that Google is planning to add a new OS to its mix. As Android Police notes, Google is already fairly well-served by Android and Chrome OS, and building a brand-new operating system from scratch takes a lot of resources, time, and debugging.