The compass symbolizes the collection of advances that combined to make long distance travel over the ocean possible. This allows you to build Cogs to trade and fight across the oceans, and Fisheries to increase the Food for your coastal cities.
The first primitive magnetic compasses in ships are mentioned in Chinese accounts dating to 83 AD, and were definitely in use by 605 AD by Arab sailors. By 850 AD they were common on Chinese ships, and by 1187 AD they had reached Europe. There the original common ’floating compass’ was perfected in 1290 AD when Amalfi added a Compass Card to it so that fine gradations of degrees from true north could be determined from the compass. Although Viking knorrs and longboats and Melanesian double-hulled canoes had traveled the open ocean before, they were all limited in their ability to consistently find their destination without a compass: Iceland, Greenland, and Vinland (North America) were all discovered by Viking sailors by accident while trying to reach someplace entirely different. With working magnetic compasses, sailors could accurately guide themselves by Magnetic North across open water without regard to landmarks. Combined with the new navigation instrument, the Viking round ship or Knorr was expanded with a deck into the Cog, the first economical ocean sailing vessel. Small cog-type ships also opened up new fishing grounds off the coasts of the North and Baltic Seas. Fishermen sailed so far in search of catch that they were fishing off the New England coast of North America by the early 16th century, almost 100 years before any colonists had reached it.