Legion

In short: The environment where user code is running within the operating system.

It's what you read on the label. A space for the user. This space is provided by the operating system. At its core; the kernel you are about to write.
This is extremely basic and low level. Within user space you will, for example, have access to files and (if done correctly) you will not have to worry about allocating physical memory or talking directly to hardware (like disk or USB or what have you). The only reason you have this access is that the operating system have defined what the file system is. Any call to reference the file system will be a call to the operating system to do the work of figuring our what the user space code wants. This is the part we will create.

What you have enbarked upon is the most basic and low level functionality of a computer.