What does FreeRTOSConfig.h control, and which settings matter most?

FreeRTOSConfig.h is the per-project header that tunes the kernel at compile time. The settings with the biggest impact: configTICK_RATE_HZ sets the tick interrupt frequency (e.g., 1000 Hz for a 1 ms tick), trading time resolution and round-robin granularity against overhead. configMAX_PRIORITIES sets how many priority levels exist. configMINIMAL_STACK_SIZE is the baseline stack you size tasks against, and per-task stack sizing is critical because stacks are a major RAM cost and overflow silently without an MMU. configUSE_PREEMPTION chooses preemptive vs cooperative scheduling, and configUSE_TIME_SLICING controls round-robin among equal priorities. configCHECK_FOR_STACK_OVERFLOW enables the overflow hook (essential during development). configUSE_IDLE_HOOK/configUSE_TICK_HOOK enable hooks for low-power sleep and periodic work. And on Cortex-M, configMAX_SYSCALL_INTERRUPT_PRIORITY defines the priority threshold below which ISRs may call kernel APIs, a correctness-critical setting. There's also the memory configuration: whether dynamic allocation, static allocation, or both are enabled, and which heap implementation is linked. I'd also wire up configASSERT to a breakpoint/handler in development because the kernel asserts on many misuse conditions. So the header is where you set tick rate, priorities, stack baseline, scheduling policy, debug hooks, the interrupt-priority threshold, and memory strategy, and the most bug-prone of these is the interrupt-priority threshold, with stack sizing the most RAM-impactful.