門 (mén) means "door" or "gate." It's the traditional form; the simplified version is 门, which keeps the same outer shape but strips the inner detail. 門 is itself a radical — one of the classic pictographic radicals, originally drawn to look like a pair of double doors swinging on hinges. You'll see it used as a component in many characters related to enclosures or openings, such as 問 (wèn, to ask — imagine a mouth 口 at the gate), 開 (kāi, to open), and 閃 (shǎn, to flash, like something darting past a doorway).

As a standalone word, 門 shows up in compounds like 門口 (ménkǒu, doorway/entrance) and 大門 (dàmén, main gate/front door). It also works as a measure word for academic subjects or skills, as in 一門課 (yī mén kè, "one course/class") or 一門手藝 (yī mén shǒuyì, "a trade/craft").

One thing to watch: in mainland China you'll almost always encounter 门 (simplified), while 門 (traditional) is standard in Taiwan, Hong Kong, and older texts. Both are pronounced identically — mén, second tone.