This is the 584th most frequent Japanese word.
なので
"なので" means "because" or "so" (used to indicate reason or cause).
In this sentence, 'なので' links the cause ('today is a holiday') with a suitable outcome ('I will go out with my friends').
今日は休日なので、友達と遊びに行きます。
Because today is a holiday, I will go out with my friends.
Here, 'なので' connects a reason ('he is out') to its resultant state ('nobody is at home').
彼は外出しているので、家に誰もいません。
As he is out, nobody is at home.
Here, 'なので' is used to connect a reason ('I am a student') to a consequent action ('I need to study').