That's why / Therefore
This expression is used to explain a reason for something or to denote a conclusion.
Examples
遅れたのは道が混んでいたわけだ。
That's why I was late; the road was congested.
彼女が出られなかったのは体調が悪かったわけだ。
The reason she couldn't come was that she was sick.
その映画は人気だから、混んでいるわけだ。
It's crowded because the movie is popular.
Quick check
その映画は人気だから、混んでいる___。