This is the 92nd most frequent Korean word.
때문이다
"It is because" or "the reason is."
In this sentence, '때문이다' indicates the reason or cause for an action described earlier. Here, it explains the reason for being late.
내가 늦게 온 것은 교통 체증 때문이었다.
The reason I arrived late was due to traffic.
The term '때문이다' explains the cause behind an event or occurrence blamed on the weather.
행사가 취소된 것은 날씨 때문이라고 들었다.
I heard that the event was canceled due to the weather.
Here, '때문이다' identifies the underlying factor for the subject's success, attributing it to consistent effort.