What are GameTiles?

I came across this line of code here:

public static GameTile[,] Tiles = new GameTile[BoardSize, BoardSize];

And was curious what it was referencing. I can’t find anything for Unity (or C#) on Google related to GameTile. I’ve seen multi-dimensional arrays initialized before like such:

private static Transform[,] grid = new Transform[width, height];

or

string[,] array2Db = new string[3, 2] { { "one", "two" }, { "three", "four" }, { "five", "six" } };

where I had presumed the word before [,] was the object or class that made up the array, but in the case of GameTile, I can’t find references to it anywhere. Thank you advance for your help and elaboration.

It is probably a class the user made himself (not part of unity) like:

public class GameTile
{

}

In the “Converting from board to world space” section of the link you refer to, you can see him building the GameTile class himself.