The pencil raft model is used in introducing students to the idea of grain and grain direction, and it seems to be an effective tool.
As you can imagine, the raft is much stiffer in the direction of its length. To get it to bend in that direction it is necessary to bend pencils. This is the grain direction.
It is much more flexible across its width. To get it to bend in that direction, one must only bend rubber bands. This is called the cross direction.
In practice however, materials aren't generally as obvious about revealing their grain direction as the pencil raft is. In fact, sometimes it's very difficult to determine by flexing a piece of the material. So it's always a good idea to mark the grain direction on all scraps of such materials that are to be kept. A plain pencil mark running parallel to the grain is plenty.
The image above illustrates the meaning of the terms grain long and grain short. The grain direction is indicated by the red arrows. As you can see, these terms simply refer to the orientation of the grain direction in relation to the dimensions of a piece of material, specifically whether the grain runs the larger or smaller of the two.