Real time in this context means software that always run in the same time. Normal server and desktop OSes such as Mac, Linux, and Windows have multitasking without exact scheduling, making it impossible to say exactly how long time it will take for a piece of code to run. In a real time OS, the time it will take a piece of code is always the same.
This is used in space craft, aircraft and similar areas.
Not to be confused with real time processing speed, eg. encoding video in real time means to encode it as fast as the frames are coming.
solved What does the term “Real-Time Software Development refer to?