An alphanumeric grid (also known as atlas grid[1]) is a simple coordinate system on a grid in which each cell is identified by a combination of a letter and a number.
[2] An advantage over numeric coordinates such as easting and northing, which use two numbers instead of a number and a letter to refer to a grid cell, is that there can be no confusion over which coordinate refers to which direction.
As an easy example, one could think about battleship; simply match the number at the top to the number on the bottom, then follow the two lines until they meet in a spot.
Algebraic chess notation uses an alphanumeric grid to refer to the squares of a chessboard.
[3] Some kinds of geocode also use letters and numbers, typically several of each in order to specify many more locations over much larger regions.