It simply means that in xTaskCreate the memory required for the task stack is allocated taken from FreeRTOS heap as documented and if configured i.e. configSUPPORT_DYNAMIC_ALLOCATION is enabled.
Besides looking up the well documented FreeRTOS sources there is a lot of good documentation e.g. FreeRTOS Kernel Developer Docs recommended to read.
See the chapters Static Vs Dynamic Memory Allocation and Memory Management there.