The Earth has two poles and at an equal distance between them lies a horizontal axis dividing the planet into the Northern and Southern hemispheres. This axis is called the Equator and is the basis of latitude measurements. Latitude is the angular distance (measured in angle degrees) between a specific location on Earth and the Equator. In essence, latitude values express the position of other horizontal axes -- called lines of latitude -- above or below the Equator.
Think of the Equator as one of the angle's rays. The vertex of a latitude angle is always the globe's center when viewed as a circle from afar. The angle's second ray is the one starting from the globe's center and reaching your location. All regions above the Equator have positive latitude values, while the regions below the Equator are given negative values. The extreme values of latitude are +90 degrees for the North Pole and -90 degrees for the South Pole.
The latitude belt is the area between two latitude lines or around -- not only on -- a single latitude line. Therefore, defining a latitude belt means calculating the value -- in angle degrees -- of its north and south ends or its single line. For example, you can say that an area is within a latitude belt of 55 to 75 degrees South or around the 25 degrees North latitude belt.
You can use latitude belts when you want to describe large regions without being specific about a position. This is especially helpful for meteorologists, who cannot describe the climate of a specific location, such as southwest Dallas as opposed to the southeastern portion of the city. However, it is helpful and accurate enough to say "low-latitude belt of South China" or that "trade winds blow away from the 30 degrees latitude belt."