What is the memory map?. What it does ?. Why this is used. I have no idea ?. Totally I don't know...
A memory map should tell you how memory is used, by telling you where your variables are located and how much memory each one uses, and by telling you where your code is located - function by function - and how much memory each function uses.
Another use of the phrase is ‘memory mapped I/O’ where the processor’s i/o ports are ‘mapped’ into reserved memory addresses.
Do you have the sentence where it used?
It depends on context. For example, the memory map of a microcontroller shows how the CPU registers, IO registers, RAM, EEPROM, and FLASH fit into the address space used by the appropriate instructions. (not all microcontrollers have all of those memory types, nor are they always in the same address space.)
The memory map of a program shows the relative location and size of the program's data variables and/or program functions, and other things that might matter to the program (stack, heap, etc.)
The phrase "memory map" may mean several different things. There is no "the memory map", unless there is. Use of the definite article usually means that there is a context explaining which "the" we are talking about.