Latitude measures a location's position on Earth between the north and south poles. Because countries are larger than a single point, the center point of a country is used. This system is imperfect, and it sometimes results in several points of latitude arguably being the best point to use. For example Palestine has a latitude of 31°25'N at the Gaza Strip, but the West Bank could also be used and has a latitude of 32°00'N.
DR Congo is right on the equator with a latitude of 0°00' N. Because of the tilt of the world's axis, countries on the equator are hot all year long. This would be the case for DR Congo as well as Nauru and Sao Tome and Principe. Nauro has a latitude of 0°32'S, making it the country farthest from the South Pole in the Southern Hemisphere. Sao Tome and Principe has the same latitude as Uganda and Kenya. All have a latitude of 1°00', making them the countries farthest from the North Pole above the equator.
With a latitude of 72°00'N, Greenland is the country farthest from the equator in the Northern Hemisphere. It is followed by Iceland with a latitude of 65°00'N and Finland with a latitude of 64°00'N. In the Southern Hemisphere, the Falkland Islands are the farthest away from the equator with a latitude of 51°45'S. Next is New Zealand with a latitude of 41°00'S.
Countries tend to be more frequent closer to the equator. Part of the reason is that the poles are cold and inhospitable. They aren't the sort of environment that would lead to a thriving nation-state. Even with the advanced technology we have today, it is difficult for human life to survive at the poles. The other part is that the equator is the longest line of latitude so there is simply more space for more countries the closer you get to the equator. In fact there are no countries with a central point within 18°00' of either the North or South pole.
The main unit of measure in latitude is the degree (°). Starting at the equator (0°) and moving both northward to the North Pole (90° North, written 90° N or +90°) and southward to the south pole (90° South, written 90° S or ?90°).