The core feature of an RTOS (Real Time Operating System) is to have a single CPU run independent units of execution, called tasks, with real-time performance. This part of an RTOS is called a multitasking executive, or a multitasking kernel. An RTOS lets the CPU run a user task until some conditions occur, at which point the CPU control is passed to another task. In this manner, multiple tasks can be run on a single CPU. If the frequency of task-switching is fast enough, it would appear as if all the tasks are running concurrently.
In REXIS, a task is just a regular function written in C, C++, or even assembly, typically with an infinite loop as the body of the function.