Related to naval warfare, it is a line across these countries. On the sea floor along that line, NATO has place sonar buoys, creating a barrier that Soviet naval vessels are unable to cross without being detected. Thus, in a NATO/Warsaw Pact war, the Soviet navy would be hampered or prevented from entering the Atlantic and disrupting cross-Atlantic shipping by a line of NATO submarines manning the detection zone. As in World War II, protection of Atlantic shipping would be important for the resupply of Europe from America.
The GIUK line is mentioned in a few books as a significant plot element, such as Tom Clancy's Red Storm Rising and Hunt for Red October.