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The lines are the grain. This board will change size across the grain, "up and down" in this picture much, much more than with the grain, "left to right".
Wood moves so little along it's grain, that for standard furniture sized work it can be disregarded. Disregard across the grain movement, on the other hand, and the piece will pull itself apart.
That's the simple part, because it is easy to express. There is also a difference between movement of the radial and tangential faces of wood. This is darned hard to explain without showing the end grain, and is not nearly as dramatic a difference (depending some on species) as the length/width difference.
What it comes down to is that a "perfect" cube cut of wood will take different shapes as humidity changes. Sometimes it will be wider than long. Sometime looking at the end grain, is won't even be rectangular.
Different species are better or worse. One of the best is "true" Mahogany. Even better is Mesquite, but finding pieces of any large size is difficult and very expensive.