(A similar method is used for binary heaps).

The sum of the root vertex at index 1, the sums of its two child vertices at indices 2 and 3, the sums of the children of those two vertices at indices 4 to 7, and so on.

With 1-indexing, conveniently the left child of a vertex at index $i$ is stored at index $2i$, and the right one at index $2i + 1$.

Equivalently, the parent of a vertex at index $i$ is stored at $i/2$ (integer division).

This simplifies the implementation a lot.

We don't need to store the structure of the tree in memory.