Thin, slender, or narrow in width or size (used for objects, body shapes, lines, etc.).
彼女は顔が小さくて首が細い。
She has a small face and a slender neck.
細い道を車で通るのは大変だ。
It's difficult to drive down a narrow road.
鉛筆の先が細いと細かい字が書ける。
If the pencil tip is fine, you can write small, detailed characters.
その川は細いので橋を渡るのに注意が必要だ。
The river is narrow, so you need to be careful when crossing the bridge.
Slang (in billiards): used to indicate being unlucky (an unfavorable situation).
ビリヤードでは相手が『細い』と言ったら運が悪いときの表現だ。
In billiards, if someone says 'hosoi', it's an expression used when things are unlucky.