Because
The Reason Is
conjunction
This is like saying "when it comes to the 何故," which means you're giving a reason, making this because or the reason is, followed by… whatever you wanna say!
Note that this is a quite formal way to say "because," so you'll mostly encounter this in writing or formal speech, and almost always at the beginning of a sentence in Japanese.
This reading is an exception but you learned them already because they're the same as the word 何故, which you learned a few levels ago. It just has some kana attached to it!
何故なら、明日がカードの締め日だからです。
The reason is that tomorrow is the cut-off date for my credit card.
理由はシンプルで、何故なら美味しいからです。
The reason is simple: it's because it tastes good.
何故なら、待ち時間は有益に使った方がいいからです。
It's because you should use your waiting time more productively.
何故なら、彼は魔法使いだったのです。
Because, he was a wizard!