A nautical mile is based on the circumference of the Earth. If you cut the Earth in half at the equator, pick up one of the halves and look at the equator as a circle. Divide that circle into 360 degrees. Then divide a degree into 60 minutes. A minute of arc on the planet Earth is one nautical mile. Because this takes in to account the arc of the Earth, it is used in air and sea travel. A nautical mile is 1,852 metres, 1.852 kilometres, 1.1508 miles, or 6,076 feet.
The nautical mile is used by sea and air navigators because of its convenience when working with charts and maps. Most nautical charts are constructed on a scale that varies from the equator to 80° north or south latitude. This means it is difficult to show a single linear scale for use on charts (realistically on scales smaller than about 1/80,000). This isn’t practical for more accurate navigation using lower scale charts.
Since a nautical mile equals a minute of latitude, it is easy to measure a distance on a chart with dividers, using the latitude scale on the side of the chart directly to the east or west of the distance being measured. Being used to working with miles and kilometres means that this sounds complicated, but for the purpose of working with charts and maps it is far easier for the navigators to use.