There is a lot of information on the web about Real Time Operative Systems, but i will be very fast because i am not an expert nor a genius in this and other themes.
This is a combination of software and hardware requirements to do real time applications. Like computers that have an Operative System (not RT) it is a built in, non visual real time system. Depending on the application, the RTOS will be a bless or your doom because the debug and implementation of this types of systems are more complex, but literally it is more efficient and faster to do something in a RTOS layer that a non-RTOS system.
In this entry i will explain RTOS basics and will finish with a video of implementing the simple example with Keil RTX CMSIS-RTOS compliant on the FRDM-K64F board for just setting and then blink a LED.