What is the result of NULL + int?

I have seen the following macro being used in OpenGL VBO implementations:

#define BUFFER_OFFSET(i) ((char *)NULL + (i))
//...
glNormalPointer(GL_FLOAT, 32, BUFFER_OFFSET(x));

Could you provide a little detail on how this macro works? Can it be replaced with a function? More exactly, what is the result of incrementing a NULL pointer?