Why are children nodes are 2*i+1 and 2*i+2?

Question

I'm currently trying to implement Huffman's compression algorithm using a Priority Queue (Min Heap). I've found a resource online that could help me, although there is a piece of code I do not understand, and unfortunately I do not find any mandatory explanation about it.

1) In function minHeapify why is left/right equal to 2*idx+1 / 2*idx +2 ?

2)In function insertMinHeap, why are we testing if `Node->frequency < minHeap->array[(i-1)/2]. What is the purpose of (i-1)/2 ?

Any kind of explanation is greatly welcomed. Appreciate your time.

void minHeapify(struct MinHeap* minHeap, int idx) 

{ 

    int smallest = idx; 
    int left = 2 * idx + 1; 
    int right = 2 * idx + 2; 

    if (left < minHeap->size && minHeap->array[left]-> 
freq < minHeap->array[smallest]->freq) //
        smallest = left; 

    if (right < minHeap->size && minHeap->array[right]-> 
freq < minHeap->array[smallest]->freq) 
        smallest = right; 

    if (smallest != idx) { 
        swapMinHeapNode(&minHeap->array[smallest], 
                        &minHeap->array[idx]); 
        minHeapify(minHeap, smallest); 
    } 
} 

void insertMinHeap(struct MinHeap* minHeap, 
                   struct MinHeapNode* minHeapNode) 

{ 

    ++minHeap->size; 
    int i = minHeap->size - 1; 

    while (i && minHeapNode->freq < minHeap->array[(i - 1) / 2]->freq) { 

        minHeap->array[i] = minHeap->array[(i - 1) / 2]; 
        i = (i - 1) / 2; 
    } 

    minHeap->array[i] = minHeapNode; 
}