The kernel needs to know the system memory map to initialize its memory management subsystem at early boot. The memory map defines the physical memory layout, which specifies how much memory is available on the system and which parts of the physical memory are reserved for memory-mapped I/O or other special purposes. The kernel uses the memory map to setup its internal memory management data structures in regions of the memory that are safe to use.
System memory is typically initialized by the firmware, which has most information about hardware in general. The firmware therefore knows the system memory map. The kernel has two methods for obtaining the memory map: the kernel can query the firmware for a memory map or the kernel can ask the boot loader to provide it. Both methods are used by kernels, usually depending on which machine architecture they're running on.