This is the 8842nd most frequent Russian word.
почему
"Почему" means "Why."
This usage of 'почему' is to ask the reason or cause behind an action or situation.
Почему ты не пришёл вчера?
Why didn't you come yesterday?
In this context, 'почему' introduces a dependent clause expressing curiosity about a particular reason.
Интересно, почему звёзды светят ночью.
I wonder why the stars shine at night.
Here, 'почему' is part of an explanatory subordinate clause elucidating a specific decision.