its called as array index.
syntax:
data type var name[size]
int a[3]
its used to allocate the memory for the variables.
0 1 2
100 102 104
these(0,1,2)is your memory and (100,102,104)is your memory address.
note:
Array index starts with the value 0 and ends with the value n-1 which n is the size.