FreeRTOS has a well-structured documentation which you can find online. Some essential topics can be found on the listed links below.

Link	Description
https://www.freertos.org/RTOS.html	Introduction into the FreeRTOS Kernel.
https://www.freertos.org/features.html	Operating principle of the FreeRTOS Kernel, addresses topics such as Tasks, Queues, Semaphores, Mutexes, Software Timers, Memory Management, Stack Overflow Protection, etc.
https://www.freertos.org/a00106.html	FreeRTOS API Reference, includes code examples for kernel configuration and FreeRTOS objects (Tasks, Queues, Semaphores, Mutexes, Software Timers, Event Groups, Buffers, etc).
https://www.freertos.org/kernel/secondarydocs.html	FreeRTOS Kernel Secondary Docs (Idle Task, Hooks, MPU, Low-Power Support, Trace Features, Run Time Stats, concept of Multitasking, Context Switching and Scheduling, etc.).