A phrase used to introduce the reason for something; literally "the reason is that..." and is followed by the cause or explanation.
彼は今日来られなかった。なぜかというと、急に用事が入ったからだ。
He couldn't come today. The reason is that an urgent matter came up.
この製品は安い。なぜかというと、品質にばらつきがあるからだ。
This product is cheap. The reason is that there are variations in quality.
試験の点数が上がった。なぜかというと、毎日コツコツ勉強したからだ。
My exam scores improved. The reason is that I studied steadily every day.