牢固 (láo gù) means 'firm, secure' — something fixed tightly and not coming loose, like a knot or a foundation. It differs from 坚固 (jiān gù, 'sturdy, solid'), which emphasizes the strength of the material itself, and from 坚实 (jiān shí, 'solid, substantial'), which stresses being well-built and reliable. 牢固 is often used for fastenings, ties, and relationships.