In wood, the grain is the pattern of lines you can see — the way the tree grew. A piece of wood is cross-grained when those lines run across it, not along its length. That kind of board is harder to cut and plane smoothly, and it may twist or split more easily as it dries. Carpenters watch for it because a cross-grained piece does not behave like a straight-grained one. The word is about the direction of the lines in the wood, not about whether the wood is “good” or “bad” in a moral sense. (You may also hear an older, rarer use about a person who is awkward and hard to please. That is not this meaning.)