A conjunction (used after nouns and na-adjectives) meaning 'because' or 'so'. A slightly more explanatory and sometimes more polite alternative to '〜だから'.
今日は休みなので、家でゆっくりします。
Since today is a day off, I'll relax at home.
彼は医者なので、健康について詳しい。
Because he is a doctor, he is knowledgeable about health.
この店は混んでいるので、予約したほうがいいです。
Since this shop is crowded, it's better to make a reservation.
彼女は有名人なので、街でよく声をかけられる。
Because she's famous, people often call out to her on the street.