The noun 大门 (dàmén) means "main entrance," "front gate," or "main door." It combines 大 (big) and 门 (door/gate) to describe the primary point of entry to a building, compound, or institution. It can also be used figuratively to mean "gateway."
学校的大门前停了一排自行车。
xuéxiào de dàmén qián tíng le yī pái zìxíngchē.
A row of bicycles was parked in front of the school gate.
请从大门进去,然后往左拐。
qǐng cóng dàmén jìnqù, ránhòu wǎng zuǒ guǎi.
Please enter through the main gate, then turn left.
这次比赛为他打开了职业生涯的大门。
zhè cì bǐsài wèi tā dǎkāi le zhíyè shēngyá de dàmén.
This competition opened the door to his professional career.