It’s a real time operating system for for serious embedded systems where standards compliance and predictability are critical — think satellites or military drones. It compares to VxWorks if you are familiar with it
> RTEMS is an open source Real Time Operating System (RTOS) that supports open standard application programming interfaces (API) such as POSIX. RTEMS stands for Real-Time Executive for Multiprocessor Systems. It is used in space flight, medical, networking and many more embedded devices. RTEMS currently supports 18 processor architectures and approximately 200 Board support packages.