Android, for phones and tablets, and Chrome OS, for laptops, are the active operating systems that Google currently offers. Both are based on Linux and have been used to power consumer devices for years, with Android first released on a phone in 2008 and Chrome OS released in 2011 after initially being announced in 2009. Now Google is working on another operating system called Fuchsia. It's open source, with the code available in GitHub for any developers that want to try it out. It's also based on an entirely different set of code from both Android and Chrome OS.
The interesting promise for Fuchsia is that it is being designed to run on any device. So the same or similar operating system could be running on a PC, laptop, tablet, phone, watch, or even something less powerful. The key benefit of having the same system on multiple devices is allowing them to communicate with each other more effectively to offer a consistent and unified experience for consumers.
The main driving force behind the development of Fuchsia may well be the Internet of Things. Connected home devices are hugely varied in terms of their processing power, from a simple light bulb to a full entertainment system or robotic device. Google, along with its major competitors, wants control over the protocols for how those devices will communicate with each other and Fuchsia could be a way to achieve that. The kernal of Fuchsia was originally designed for use in very low-powered devices and the Google developers assigned to the project are experts in that field. Google do already have an operating system in that arena, called Brillo, but Fuchsia far is more ambitious in it's scope and potential.
So could Fuchsia become a replacement for Android? It's certainly possible if the project gathers momentum and starts to become a viable consumer product, but there is really no way of knowing right now as these are the still the very early days of Fuchsia. Google are known for starting new projects as experiments and being relatively public about them. They then evaluate the projects over time for the potential benefits. Many Google projects have even become popular consumer products and services, only to be dropped later. It is a promising proposition to have one operating system across the widest possible range of devices, but it will be sometime before it becomes clear whether Fuchsia will be that system.