LLVM getelementptr indices use/meaning

Question

I just started learning LLVM and I am wondering why we have two indices in getelementptr? what are the first and second indices (0 and 0) used for?

@tmp = global [18 x i8] c"Hello world!: %d\0A\00"

declare i32 @printf(i8* %0, ...)

define i32 @fact(i32 %x) {
0:
    %1 = icmp sle i32 %x, 0
    br i1 %1, label %2, label %3

2:
    ret i32 1

3:
    %4 = sub i32 %x, 1
    %5 = call i32 @fact(i32 %4)
    %6 = mul i32 %x, %5
    ret i32 %6
}

define i32 @main() {
entry:
    %0 = getelementptr [18 x i8], [18 x i8]* @tmp, i32 0, i32 0 ; <---- HERE
    %1 = call i32 @fact(i32 23)
    %2 = call i32 (i8*, ...) @printf(i8* %0, i32 %1)
    ret i32 1
}
enter code here