「夜」 means night, referring to the period of darkness from sunset to sunrise. It's used to talk about evening activities, dinner, or anything happening during the night. A fundamental concept for time.

日(ひ)が沈(しず)んでから昇(のぼ)るまでの暗(くら)い時間帯(じかんたい)を指(さ)します。夜(よる)の活動(かつどう)や出来事(できごと)について話(はな)すときに使(つか)います。