牢 (láo) can mean "firm" or "secure" as an adjective, but it also commonly appears as part of the noun 地牢 (dìláo, "dungeon") or 监牢 (jiānláo, "prison"). In classical and literary Chinese, 牢 on its own can refer to a prison or pen. Context determines whether it conveys firmness or confinement.
这个笼子很牢,动物跑不出去。
Zhège lóngzi hěn láo, dòngwù pǎo bù chūqù.
This cage is very sturdy; the animals can't get out.
请把绳子系牢,别让它松开。
Qǐng bǎ shéngzi jì láo, bié ràng tā sōngkāi.
Please tie the rope tightly; don't let it come loose.
犯人被关进了地牢里。
Fànrén bèi guānjìnle dìláo lǐ.
The prisoner was locked up in the dungeon.