安全 means 'safe, secure' in a general sense — free from danger, risk, or harm. It is broader than 平安 (píng ān), which describes being 'safe and sound' after a journey or event, and more common than the adjective sense of 保险 (bǎo xiǎn 'safe, reliable'), which also means insurance. 牢固 (láo gù) refers to physical firmness like a structure.