the reason is; because — opens a new sentence explaining the one before it
この計画には反対です。なぜなら、費用がかかりすぎるからです。
kono keikaku ni wa hantai desu. nazenara, hiyou ga kakarisugiru kara desu.
I'm against this plan. The reason is that it costs too much. neutral
今日は道が混んでいる。なぜなら、近くで工事をしているからだ。
kyou wa michi ga konde iru. nazenara, chikaku de kouji o shite iru kara da.
The roads are congested today. The reason is that there's construction happening nearby. neutral
Variants
なぜならばnazenarabaliterary
keeps the classical conditional ば — found in formal writing and speeches, functionally identical to plain なぜなら.
なぜなら
なぜならnazenaraN3formal