Wood fibers grow long-ways up and down along the tree trunk. The wood is very strong this way because of the fibers. We say that wood is stronger "along the grain," which is just a different way of saying "up and down along the fibers."
As you go sideways across the tree trunk, or across the grain, the wood is not as strong.
This means that if you need really strong wood, put the wood along the grain!
Now that we've discussed wood's strength a bit, let's test a piece of wood and see how strong it is.