Imaginary lines that run east and west circling our globe. These lines measure north and south distances from the equator.
Imagine that Earth is a transparent sphere. Place one end of an imaginary straight line at the center of the Earth. Place the other end on the equator. No matter which direction you go toward the equator you are in the equatorial plane. In the equatorial plane you are at 0°.
Keep one end of the line at the center of Earth and raise the other end from the equatorial plane (It is a convention that "up" on a map or globe is north.) along the edge of the sphere. If you raise it 45°, you are at 45 degrees North latitude. Wisconsin is at that latitude. Moving around the globe tracing an imaginary circle at this angle you will also find South Dakota, Mongolia, France, and Maine. If you raise the end of the line to 90 degrees North (straight up from the center) you are at the North Pole. There is no line here.
If you lower the line while holding it at the center of the Earth, you move into the southern latitudes. Madagascar is at 20 degrees South. Moving around the Earth at 20 degrees north, you touch northern Paraguay, southern Bolivia, and Australia. To be more exact the degrees of latitude can be divided into minutes and the minutes into seconds. Of course, to locate any particular place you also need the longitude. 135 East longitude runs through the center of Australia. You can look up the location of major cities at some sites on the web.