Actually, that is close but not right. The grid_code is a hint as to how this data was created. Imagine a (huge) grid with each cell numbered consecutivly starting with 1 in the top left corner.
E.G.
1, 2, 3, 4, 5
6, 7, 8, 9, 10
11, 12, 13, 14, 15
16, 17, 18, 19, 20
21, 22, 23, 24...