From DT Online
All timber has a Grain as a result of the way the cellular structure of a tree forms wood fibres along its length.
For simplicity, the fibres can be regarded as a bundle of drinking straws: quite strong along their length but easily separated from each other. Similarly, wood can be quite strong along the grain but weak and easiy broken across the grain.
As consequence, when shapes are cut out of solid timber and not reinforced or Laminated on to a backing board for example, care must be taken to avoid areas of the shape where the grain runs across a narrow width or short distance. This weakness is known as Short Grain.
Some Manufactured Boards such as Plywood alleviate this problem by ensuring that alternate layers of Veneer have grain in different directions. When this solution is not acceptable, solid wood shapes can be reinforced by drilling through their width at right angles to the grain and inserting a length of Dowel.