A stopping point or pause when something reaches a temporary conclusion; a milestone/closure.
今日の作業はこれで一区切りだ。
Today's work is finished for now.
プロジェクトが一区切りついたら打ち上げをしよう。
Let's have a celebration once the project reaches a milestone.
章ごとに一区切りをつけて読んでいくと理解しやすい。
If you read by making a pause at each chapter, it's easier to understand.