front door; front entrance
Refers to the main or formal entrance of a building, as opposed to a back or side entrance.
お客様は表口からお入りください。
Guests, please enter through the front door.
裏口 is the back entrance of a building, while 表口 is the front entrance facing the main street.
Compound of 表 (おもて, 'front') and 口 (ぐち, 'entrance').