A conjunction used to introduce the reason for a preceding statement (often in the form '..., because...').
彼は来られません。なぜなら、急に熱が出たからです。
He can't come. The reason is that he suddenly developed a fever.
その計画は中止になった。なぜなら予算が足りなかったからだ。
The plan was canceled because there wasn't enough budget.
なぜなら、彼女はまだ経験が足りないからです。
The reason is that she still lacks experience.