The characters combine to paint a powerful and literal image: closing (闭) the border passes (关), and locking (锁) up the entire country (国). It implies an active, forceful sealing-off from the outside world, not just a passive lack of contact.
Historically, 闭关锁国 (bìguān suǒguó) refers to the foreign policy of the mid-to-late Qing Dynasty (and to some extent, the Ming Dynasty before it). Believing China to be the self-sufficient “Middle Kingdom” (中国), the emperors severely restricted foreign trade to a single port (Canton, modern-day Guangzhou) and limited contact with foreigners. The goal was to preserve cultural purity and maintain political stability by minimizing disruptive foreign influences. In the modern Chinese consciousness, this policy is almost universally seen as a catastrophic historical mistake. It's blamed for causing China to fall behind the West in science and technology, leaving it weak and vulnerable. This weakness led directly to what is known as the “Century of Humiliation” (百年国耻), where foreign powers forced unequal treaties upon China, starting with the Opium Wars.
You will almost never hear `闭关锁国` used to describe a desirable or positive action. It is a historical term used in modern contexts as a powerful criticism or a dire warning.