An expression meaning 'when it becomes (something)' or 'when it comes to (a certain time/situation)'; used to indicate what happens in that state or time.
冬になると富士山に雪が積もる。
When winter comes, snow accumulates on Mount Fuji.
話題になると、彼は急に饒舌になる。
When the topic comes up, he suddenly becomes talkative.
学期末になると宿題が増える。
When it becomes the end of the term, homework increases.