Merge is the core technology of MergeSort, that is why it was named MergeSort.

convert input array -> left_part and right_part

left_sorted_part  = MergeSort(left_part)
right_sorted_part = MergeSort(right_part)

Merge(left_sorted_part, right_sorted_part)

1. taking one (l) from left_sorted_part
2. taking one (r) from right_sorted_part
3. find the smaller(s) one in l and r
4. put s into result array
5. put bigger one back to left_sorted_part or right_sorted_part
6. loop to step 1 until one is empty