The Coder's Handbook
Sprites and Animation
Each column represents a single frame in that animation. Following left to right you can see a progression in each animation.
This means that if I reference a robot, I know that is going to be:
The third type of animation (attack)
The fourth frame in that animation sequence.
Each column represents a different type of tile. For instance, we have night and day versions of different patterns of tiles.
This means that if I reference tile, I know that it is going to be:
The third type of tile (dirt).
The fourth version of that tile (night type).
The SpriteSheet has other handy methods. Two notable ones are getVerticalCount() and getHorizontalCount() to tell you how many elements are in each sheet, so you can avoid hardcoding.