Prerequisites

You need strong understanding of recursion and binary trees. Segment trees support range queries (like sum, min, max over a range) and point updates, both in $O(\log n)$ time. They're built by recursively dividing the array.

This is an advanced data structure. Complete easier tree problems first. Each node stores aggregate information for its segment. Queries combine information from $O(\log n)$ non-overlapping segments that cover the query range.