When SELinux has to decide whether it has to allow or deny a particular action, it makes a decision based on the context of both the subject (which is initiating the action) and the object (which is the target of the action). These contexts (or parts of the context) are mentioned in the policy rules that SELinux enforces.
The context of a process is what identifies the process to SELinux. SELinux has no notion of Linux process ownership and, once running, does not care how the process is called, which process ID it has, and what account the process runs as. All it wants to know is what the context of that process is, which is represented to users and administrators as a label. Label and context are often used interchangeably, and although there is a technical distinction (one is a representation of the other), we will not dwell on that much.
Let's look at an example label: the context of the current user (try it out yourself if you are on a SELinux-enabled...