During times of war or danger, nations who fear that their country could be attacked or invaded try to seal their borders against any unwanted intruders. For landlocked countries, that means heightening passport checkpoints and strengthening fences. For countries with ocean borders, it is harder. They must set up coastal watchtowers and lookouts. If the coast is clear of any unwanted intruders, people feel relaxed and safe.

This phrase became popular for anyone wishing to express that they feel no external danger emerging from their surrounding environment. It can also mean that no one is watching you, or spying on you. For that reason, this idiom is also popular among the military or even among criminals who are trying to sneak around without police seeing them.