in overly simple terms:
if you imagine that your computer's ram is one giant big free for all array...
your operating system puts blocks around sections of it, saying 'this block is for the OS, that block is for your web browser, this one is your video card driver program, that one is your mouse, ... and so on for every program running. Allowing one program to modify the memory of another can be fun (back in the day) but for various reasons (data security, virus defense, system crash defense, more) the OS does not allow a program to access the memory that is marked as owned by another program. If you do that, the OS will KILL your program with extreme prejudice -- and give you an access violation error message. So that is what it is: you are trying to do something to memory that does not belong to you.
to trigger this, a couple of lines of code that will blow up for you to play with:
int *x = null;
*x = 1234;
y = 1234;