Latitude and longitude coordinates are part of a geographic coordinates system that can identify any location on the Earth. This system uses a spherical surface that covers the Earth. This surface is divided in a grid and each point on this surface corresponds to a specific latitude and longitude, just like each point on a cartesian plane corresponds to a specific x and y coordinates. This grid divides the surface of the Earth with two sets of lines that run parallel to the equator and from the North Pole to the South Pole.
The lines parallel to the equator, and so lines that run east to west, have a constant latitude value. They are, adequately, called parallels. The line that runs right over the equator defined the latitude value 0. Going north towards the North Pole the latitude value increases from 0 to 90 at the North Pole. New York, which is about halfway between the equator and the North Pole has a latitude of 40.71455. From the equator going south the latitude values become negative and reach -90 at the South Pole. Rio de Janeiro has a latitude of -22.91216.
The lines that run from the North Pole to the South Pole have a constant longitude value. Those lines are called meridians. The meridian which defines the longitude of value 0 passes over Greenwich in England. Going west from Greenwich, say towards the Americas, the longitude values become negative. The longitude values west of Greenwich go from 0 to -180 and the longitude values going east go from 0 to 180. Mexico City has a longitude of -99.13939 and Singapore has a longitude of 103.85211.
Latitude and longitude coordinates are for example used by GPSs. At any point in time, your current location can be precisely defined by latitude and longitude coordinates.