It is important that you understand the differences between an OS kernel and an OS utilised by an embedded system like your mobile phone. The difference is important in the sense that an OS kernel is what actually controls the computer hardware; whereas, an embedded system links the computer software together through a protocol. An OS kernel is what you would call 'built-in' - it is pre-configured and predefined by the manufacturer, whereas an embedded system connects to your computer through a protocol. So, what exactly is a kernel? In the simplest terms, a kernel is the central processing unit or CPU of a computer that executes instructions sent by the user using a computer language such as C++.
Read more about linux kernel source here.
There are two types of kernels: main kernel and userspace kernel. Main kernel functions as the brains of the whole system. All processes are run directly by the main kernel. This kind of kernel is not that different from the mainframe of your personal computer, except for the fact that it is executed directly at runtime (it is pre-compiled by the manufacturer). Userspace kernel on the other hand is what you will use to run your applications and devices such as cameras, printers etc.
With the invention of the x-platform, the software development kit or SDK, came the idea of open source kernels. These are developed for the x-platform by the device implementor or developer (the company who created the hardware) and released under an open licence, allowing anyone to utilise the kernel on any device without any charges. Nowadays there are a number of companies offering this kind of kernels, but their license agreement differs from one another. Some of them even have permissive licence, while some others require the company to take a certain number of security precautions.
Open source kernels can be freely utilised. However, it doesn't mean that you can just copy it and use it on your PC. For example, if you want to compile your own kernel using the Compaq hardware drivers, you will need the relevant source code along with the Compaq proprietary SDK. The only exception here is when you need official Microsoft Windows drivers - in such a situation the only thing you'll need is the Microsoft SDK.
Furthermore, a big disclaimer comes up if you're thinking of using this technology on your mobile phone. Mobile phones aren't really meant to use this type of software and you might end up voiding your mobile phone warranty. But if you're going to use the kernel on a tablet or similar device, you won't void your device warranty as long as you're using the proper software licence.
So, what's the big deal anyway? Well the reason why these kernels are open source is to allow freedom and competition amongst device and software developers. By letting different people utilise the kernel, the quality of the software improves, which, in turn, means the quality of the device improves. You wouldn't want to use a device which didn't have the latest version of the Android operating system - so why would you want to use a Linux kernel which hasn't been updated for years!
Plus, as these kernels are developed using the Rust programming language, you're basically guaranteed a stable system that should run across multiple devices. That means if you're an iPhone user who likes to take pictures of themselves, you'll be able to take pictures of yourself on an iPhone with an iPhone kernel and send it to other iPhones with iPhones. Likewise if you're a Linux user and like to use media players, then you'll be able to play music or videos on devices which have been developed with the Rust language. It really is a versatile program and no matter what kind of computer you're on, it should be able to utilise it.
It's also worth noting that although this technology is available for free online, the manufacturers do not offer a full range of support. This is down to their desire for total control of the system. The companies that do provide a range of support include XOOM and LinXOS. However, these manufacturers do sell a support package called Softies, which is aimed at users who want to get started with utilising the technology but are wary of getting stuck in the mud. So as long as you're prepared to take the time to investigate the various programs out there, you shouldn't have any problems.