A grain in a piece of wood is the pattern produced by the annual bands or rings of a tree during its lifetime.
Slow growing trees have tight rings, fast growing tree rings are spread out further. Another interesting aspect of tree rings is that we can also tell what the weather was like by the structure of the rings.
A hard weathered year will show irregular rings, and the grain may be straight, spiral or interlocked and wavy - while mild weathered years show uniform rings.
And you still can tell the age of a tree based on the number of rings it has.