FreeRTOS is a lightweight RTOS design for embedded systems. Distributed freely under the MIT open source license, FreeRTOS includes a kernel and a growing set of libraries suitable for use across all industry sectors. FreeRTOS is built with an emphasis on reliability and ease of use. It allows the creation of multiple 'tasks' (multi-threading) and provides essential kernel elements for the snychronization between these tasks(Things like Mutexes, Semaphores, queues, etc.) It also enables the implementation of more advanced features like Software timers, and tickless idle for power-saving purposes.
With a focus on compactness and speed of execution, FreeRTOS gives the user precision control over RTOS elements and forgoes the 'fluff' present in a full-blown OS. This allows users to scale their RTOS implementation to the needs of their project.