Why int[char*] is valid and returns char*?

Question

I found this and tried to figure out why it's valid.

int main(int ac, char* av[])  
{ return ac[av]==0; } //returns 0  

I found out that ac[av] returns char* so then when comparing it to 0 it's obvious it will return 0 .
But why can we use square brackets on integer variable?
Why int[char*] returns char*?